Implement times(2) syscall

[Max042004]

Guests calling times(2) got -ENOSYS: the syscall was absent from
dispatch.tbl and abi.h had no SYS_times entry. Shell timing builtins and
build tools that poll times() for elapsed/CPU tick accounting fall over
on the error return.

sys_times fills the four struct tms fields from host getrusage:
RUSAGE_SELF supplies tms_utime/tms_stime and RUSAGE_CHILDREN supplies
tms_cutime/tms_cstime. Guest fork() maps 1:1 to a host fork and
sys_wait4 reaps through host waitpid, so the host's children accounting
covers exactly the guest's waited-for children. The return value is host
CLOCK_MONOTONIC converted to USER_HZ (100) ticks: Linux returns
jiffies-since-boot, and POSIX only requires an arbitrary-but-fixed
reference point that successive calls can difference. Tick conversion
uses the same 100Hz that core/stack.c advertises via AT_CLKTCK, so
libc's sysconf(_SC_CLK_TCK) scaling stays consistent. A NULL buffer is
accepted as on Linux, returning only the tick count.

Testing

• New test-times (registered in manifest.txt): tick rate via
  sysconf(_SC_CLK_TCK), basic call, NULL buffer through the raw syscall,
  return value advancing across a 60ms sleep, self utime+stime growth
  against a measured 100ms CPU burn, cutime/cstime growth after reaping
  a child that burns 100ms CPU, and EFAULT on a bad pointer — 7/7.
• make check green (driver 81 passed / 0 failed / 3 skipped;
  skips are pre-existing fixture-absence).
• check-syscall-coverage passes.

---

Summary by cubic

Add the times(2) syscall so guests get correct CPU accounting and an elapsed tick count instead of -ENOSYS. Restores shell timing and tools that poll times(), and excludes emulator helper subprocess CPU from child totals.

• New Features
  • Added SYS_times to the ABI/dispatch; wired sc_times.
  • Implemented sys_times: self utime/stime from getrusage(RUSAGE_SELF); child cutime/cstime from proc accumulators updated at each guest-child reap (sys_wait4, sys_waitid, table reaper, CLONE_VFORK). Credit only on terminal exit/signal; ignore WUNTRACED, WCONTINUED, and WNOWAIT peeks; count only proc-table children, not helpers.
  • Returns CLOCK_MONOTONIC in 100 Hz ticks (matches AT_CLKTCK); accepts NULL; -EFAULT on bad pointer.
  • Added test-times: verifies _SC_CLK_TCK==100, tick advance, self and child CPU accumulation (including cstime), no credit on waitid(WNOWAIT), NULL buffer, and bad-pointer -EFAULT.

^{Written for commit 7cd2c0e. Summary will update on new commits.}

[jserv]

Rebase latest main branch and resolve conflicts.