posix: unistd: support for confstr()

Support querying POSIX string configuration values (similar to
sysconf()).

confstr() is required by the POSIX_SINGLE_PROCESS Option
Group as detailed in Section E.1 of IEEE-1003.1-2017 and has
been part of the specification since POSIX-2.

The POSIX_SINGLE_PROCESS Option Group is required for PSE51,
PSE52, PSE53, and PSE54 conformance, and is otherwise mandatory
for any POSIX conforming system as per Section A.2.1.3 of
IEEE-1003-1.2017.

With this, we have complete support for the POSIX_SINGLE_PROCESS
Option Group.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>