add CI test for Cygwin

[jeremyd2019]

• fix pathnative and pathposix for Cygwin (so it builds). I don't know why this doesn't fail in 1.0.10, something else must have been fixed
• seems bin/package can't tell the difference between cygwin.i386 and cygwin.i386-64
• bin/package make -j5 hangs
• bin/package test hangs in mamake regression tests
• number of shtests fail

Hopefully this can be a start to figure out the issues
@xeiavicaflashrepository @gisburn

[jeremyd2019]

https://github.com/jeremyd2019/ksh/actions/runs/13891921261/job/38865231310

[jeremyd2019]

I added a cygwin matrix to test 32-bit also (though 32-bit cygwin is no longer supported upstream): https://github.com/jeremyd2019/ksh/actions/runs/13906198215

[jeremyd2019]

Looking back on this now, this is wrong per the specification in the comments above, vs the documentation of cygwin_conv_path.

For cygwin_conv_path, if size is 0, buf may be NULL and it will return the size of buffer required. If size is not 0, it will return 0 on success or -1 on error (error including ENOSPC if size is too small to hold the result).