Integrate upstream dash changes into unbourne shell

[notwa]

this merges the changes from the previous commit (as listed in the source), up to the latest commit to dash, with the changes seen here:
https://git.kernel.org/pub/scm/utils/dash/dash.git/diff/?id=057cd650a4edd5856213d431a974ff35c6594489&id2=fba95e9e4a5d0f1f1ac9f7d86557e47bc0e2656c&context=5&ignorews=0&dt=0
patching was done by hand, but I've done several passes over the past day to make sure I got everything. note that I've omitted the changes from e.g. stat to stat64.

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having an Actually Portable shell is potentially useful to me. however, I was running into bugs that had already been fixed upstream — this bug in particular, involving variable expansions. but after patching that, I figured I may as well apply the rest of the changes.

there are still some quirks on Windows, but my PR hasn't affected them at all:

• only exec program checks paths for .com and .exe extensions; simply running program will not find program.exe in $PATH. is this intentional?
• stderr does not play well when run from an MSYS2 terminal, either going silent or getting involuntarily captured(?). I can't even see the whole trace in this scenario because it gets eaten:

./unbourne.com --strace -c 'echo hello >&2'

[jart]

Wow a lot of work went into this. Thank you! This is one of the first things I put in the codebase and the way everything is smooshed together sort of reflects that, since the build tools weren't as good back then.

If you're running it on Windows, then the biggest issue I've encountered so far is the newline issue. Two of us have tried to fix the \n \r\n` thing so it can work better on Windows and failed because the almquist code is nontrivial to change.

I'd love to see that fixed somehow. In the meantime I've been using o//examples/cosh.com as my shell on Windows.