headers: Implement _ISOC11_SOURCES macro when __POSIX_C_SOURCE defined

headers: Implement _ISOC11_SOURCES macro when __POSIX_C_SOURCE defined

When _ISOC11_SOURCES is defined for glibc at the same time
__POSIX_C_SOURCE is defined, it extends the __POSIX_C_SOURCE definition
by exaclty what C11 adds to the spec for each system header.  We follow
both OpenBSD's and glibc's convention by also C11 or higher compliation
mode is selected.

The Open Group is working on issuing a new version of the POSIX standard
that will realign the standard from C99 to a newer version of C. This
commit is a stop-gap measure for greater compatibility until that
environment has been standardized.

Reviewed by:		brooks@, arichards@, Olivier Certne
			(comments tweaked before commit)
PR:			255290
Sponsored by:		Netflix
Differential Revision:	https://reviews.freebsd.org/D29902