posix: implement confstr()

[cfriedt]

This is part of the See POSIX Roadmap for LTSv3 (RFC #51211).

confstr() is required as part of POSIX_SINGLE_PROCESS Subprofiling Option Group.

For more information, please refer to https://pubs.opengroup.org/onlinepubs/9699919799/functions/confstr.html

[moonlight83340]

Any tips where to begin ? I have read the PR for sysconf. (#67500)
I think of conftstr a switch case function like but not sure if we could do it differently.

_CS_PATH
_CS_POSIX_V7_ILP32_OFF32_CFLAGS
_CS_POSIX_V7_ILP32_OFF32_LDFLAGS
_CS_POSIX_V7_ILP32_OFF32_LIBS
_CS_POSIX_V7_ILP32_OFFBIG_CFLAGS
_CS_POSIX_V7_ILP32_OFFBIG_LDFLAGS
_CS_POSIX_V7_ILP32_OFFBIG_LIBS
_CS_POSIX_V7_LP64_OFF64_CFLAGS
_CS_POSIX_V7_LP64_OFF64_LDFLAGS
_CS_POSIX_V7_LP64_OFF64_LIBS
_CS_POSIX_V7_LPBIG_OFFBIG_CFLAGS
_CS_POSIX_V7_LPBIG_OFFBIG_LDFLAGS
_CS_POSIX_V7_LPBIG_OFFBIG_LIBS
_CS_POSIX_V7_THREADS_CFLAGS
_CS_POSIX_V7_THREADS_LDFLAGS
_CS_POSIX_V7_WIDTH_RESTRICTED_ENVS
_CS_V7_ENV

_CS_POSIX_V6_ILP32_OFF32_CFLAGS
_CS_POSIX_V6_ILP32_OFF32_LDFLAGS
_CS_POSIX_V6_ILP32_OFF32_LIBS
_CS_POSIX_V6_ILP32_OFFBIG_CFLAGS
_CS_POSIX_V6_ILP32_OFFBIG_LDFLAGS
_CS_POSIX_V6_ILP32_OFFBIG_LIBS
_CS_POSIX_V6_LP64_OFF64_CFLAGS
_CS_POSIX_V6_LP64_OFF64_LDFLAGS
_CS_POSIX_V6_LP64_OFF64_LIBS
_CS_POSIX_V6_LPBIG_OFFBIG_CFLAGS
_CS_POSIX_V6_LPBIG_OFFBIG_LDFLAGS
_CS_POSIX_V6_LPBIG_OFFBIG_LIBS
_CS_POSIX_V6_WIDTH_RESTRICTED_ENVS
_CS_V6_ENV

There are a lot of define, not sure of value that shoudl get.
Maybe I should try from basic implementation with return control and then go to a full implementation with the case I can match.

[cfriedt]

The constants could probably be an enumeration in unistd.h

The function itself should do some basic error checking (valid values of int name). If an invalid name is passed, it should return 0 and set errno to EINVAL.

Other than that, it should return 0 and not modify errno.

[moonlight83340]

I think #70274 close that issue.