Linux: Use in-tree copy of SO_ constants for !__USE_MISC [BZ #24532]

Linux: Use in-tree copy of SO_ constants for !__USE_MISC [BZ #24532]

The kernel changes for a 64-bit time_t on 32-bit architectures
resulted in <asm/socket.h> indirectly including <linux/posix_types.h>.
The latter is not namespace-clean for the POSIX version of
<sys/socket.h>.

This issue has persisted across several Linux releases, so this commit
creates our own copy of the SO_* definitions for !__USE_MISC mode.

The new test socket/tst-socket-consts ensures that the copy is
consistent with the kernel definitions (which vary across
architectures).  The test is tricky to get right because CPPFLAGS
includes include/libc-symbols.h, which in turn defines _GNU_SOURCE
unconditionally.

Tested with build-many-glibcs.py.  I verified that a discrepancy in
the definitions actually results in a failure of the
socket/tst-socket-consts test.