posix: Move POSIX configs out of experimental

Many out of the POSIX subsystem configs are enabled automatically
when merely CONFIG_POSIX_API is enabled, which is a prerequisite for
many networking samples. This causes a massive experimental warning
printout when building with warnings enabled.

Since the new POSIX Kconfig configuration options are already present
in Zephyr for 2 release cycles and seem settled, I suggest we move
them out of experimental phase.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>