posix: deprecate POSIX_SIGNAL in favour of POSIX_SIGNALS

This change deprecates CONFIG_POSIX_SIGNAL in favour of
the plural CONFIG_POSIX_SIGNALS, which maps directly to the
name of the standard POSIX Option Group POSIX_SIGNALS.

Additionally, mark signals as experimental.

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