Enable termios stdlib module

[ada-x64]

Closes #302.

Enables the termios stdlib module on the Nanvix port.

What changed

• Turn termios on — drop it from the "not available" list in
  configure.ac (and mirror in the generated configure).
• Patch around missing libc functions — Nanvix's libposix only
  exports tcgetattr / tcsetattr (as no-op stubs). The other eight
  POSIX termios functions (tcdrain, tcflush, tcflow,
  tcsendbreak, cf{g,s}et{i,o}speed) are missing and would fail
  to link. Following the HAVE_MSYNC pattern from Enable the mmap stdlib module on Nanvix #561:
  • add each function to AC_CHECK_FUNCS,
  • add matching HAVE_* slots in pyconfig.h.in,
  • guard each call site in Modules/termios.c — cf*speed calls
    fall back to zero/no-op so tc{get,set}attr still works
    end-to-end; the four top-level functions raise termios.error with
    errno = ENOSYS, matching the errno-based convention used
    elsewhere in the module.
• Skip four PtyTests methods on standalone mode — see below.

On any platform with the full termios surface (i.e. every existing
in-tree target other than Nanvix), the HAVE_* macros are defined and
behaviour is unchanged.

Why the test_builtin.py skips

Enabling termios exposes a pre-existing standalone-mode gap. The
chain:

• Before this PR, import termios failed on Nanvix.
• That made import tty fail (it does from termios import *),
  which made import pty fail.
• test_builtin.py does try: import pty / except ImportError: pty = None
  and gates PtyTests on @unittest.skipUnless(pty, ...), so all
  four methods skipped silently.
• After this PR, termios → tty → pty all import cleanly, the
  gate opens, and the four methods actually run for the first time.
• They then fail on standalone because they call os.pipe(), which
  is unimplemented in standalone mode (tracked as NSKIP023 /
  [posix] Support pipe() nanvix#343).

The four methods get @unittest.skipIf(support.is_nanvix_standalone, ...)
matching the existing pattern in test_genericpath.py and
test_posix.py. Single-process and multi-process modes have working
pipe() and run the tests normally.

Verification

./z distclean && ./z clean && ./z --mode=$MODE setup && ./z test
green across all three modes:

mode	result
standalone	116/116
single-process	121/121
multi-process	121/121

Smoke import termios; print(termios.B9600) prints 9600 per mode
against the staged ramfs.

Follow-ups (separate PRs)

• nanvix/nanvix umbrella for the eight missing termios functions.
• nanvix/cpython cleanup ticket to drop the HAVE_* guards once
  libposix grows the symbols (analogue of [build] Remove HAVE_MSYNC guard in mmapmodule.c once nanvix/nanvix#2212 lands #562).
• NSKIP for the no-op tc{get,set}attr runtime semantics — they link
  but do nothing.

[Copilot]

Pull request overview

Enables the termios stdlib extension module for the Nanvix port by removing it from Nanvix’s “not available” module set and adding autoconf-based feature detection/guards so the module links even when Nanvix’s libc lacks parts of the POSIX termios surface.

Changes:

• Enable termios on Nanvix by removing it from PY_STDLIB_MOD_SET_NA (and mirroring in generated configure).
• Add AC_CHECK_FUNCS probes + pyconfig.h.in templates for missing termios-related libc functions, and guard the affected call sites in Modules/termios.c.
• Skip PtyTests input/pty-driven tests on Nanvix standalone mode due to os.pipe() returning ENOSYS.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
pyconfig.h.in	Adds #undef HAVE_* templates for termios-related libc function probes.
configure.ac	Adds AC_CHECK_FUNCS entries for termios funcs and enables the termios module for Nanvix.
configure	Mirrors configure.ac changes in the checked-in generated script.
Modules/termios.c	Guards calls to missing termios APIs; provides fallbacks/errors when unavailable.
Lib/test/test_builtin.py	Skips pty-based input() tests on Nanvix standalone where os.pipe() is unimplemented.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[ppenna]

Could you file issues for those in nanvix repo?

[ada-x64]

Filed in nanvix/nanvix — one per missing or stubbed function:

Missing from libposix (guarded behind HAVE_* in this PR):

• nanvix/2220 — tcdrain
• nanvix/2221 — tcflush
• nanvix/2222 — tcflow
• nanvix/2223 — tcsendbreak
• nanvix/2224 — cfgetispeed
• nanvix/2225 — cfgetospeed
• nanvix/2226 — cfsetispeed
• nanvix/2227 — cfsetospeed

No-op stubs in dummy.rs (link, silently lie — separate behavioural bugs):

• nanvix/2228 — tcgetattr
• nanvix/2229 — tcsetattr

Each issue includes the POSIX signature, where it's needed, the
current cpython-side workaround, acceptance criteria, and a
revert checklist for when it lands.