posix: deprecate POSIX_ENV _CONFSTR _SYSCONF and _UNAME

This change deprecates the following Kconfig options
* CONFIG_POSIX_ENV
* CONFIG_POSIX_CONFSTR
* CONFIG_POSIX_SYSCONF
* CONFIG_POSIX_UNAME

and instead groups them into a single Kconfig option that maps
directly to the standard. Namely, CONFIG_POSIX_SINGLE_PROCESS.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>