posix: deprecate POSIX_FNMATCH GETOPT GETENTROPY

The functions fnmatch(), getopt(), getentropy()
and others are grouped into the standard Option Group
POSIX_C_LIB_EXT.

The getentropy() function is currently in-draft for
Issue 8 as of 2021.

https://www.opengroup.org/austin/docs/austin_1110.pdf



Not surprisingly, the POSIX_C_LIB_EXT Option Group
also includes the highly debated strnlen() function.

Moving that function will be deferred until later.

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