Installed-header hygiene (BZ#20366): Simple self-contained fixes.

Some headers did not include all of their prerequisite headers.

	* rpcsvc/nislib.h: Include rpcsvc/nis.h.
	* sysdeps/unix/sysv/linux/netrose/rose.h:
	Include sys/socket.h and netax25/ax25.h.

<endian.h> only defines BYTE_ORDER, BIG_ENDIAN, LITTLE_ENDIAN,
etc. under __USE_MISC; glibc's headers should use __BYTE_ORDER,
__BIG_ENDIAN, __LITTLE_ENDIAN, etc. instead.

	* inet/netinet/icmp6.h, inet/netinet/ip6.h
	* resolv/arpa/nameser_compat.h:
	Use __BYTE_ORDER etc. instead of BYTE_ORDER etc.

sys/types.h only conditionally defines caddr_t and clockid_t.

	* sysdeps/unix/sysv/linux/sys/quota.h:
	Use __caddr_t instead of caddr_t.
	* sysdeps/unix/sysv/linux/sys/timerfd.h:
	Use __clockid_t instead of clockid_t.

Remove a #warning that was the sole actual problem with using sys/ipc.h
without _GNU_SOURCE/_XOPEN_SOURCE.

	* sysvipc/sys/ipc.h: Remove unnecessary #warning.

_LIBC, __USE_XOPEN2K8, and __STDC_VERSION__ are not always defined.

It seems to me that _LIBC should not appear in installed headers, but
avoiding that for argp specifically would require more surgery than
feels appropriate for this patch set.  It's possible that
"#ifdef _LIBC" would be sufficient, but I wanted to be conservative.

All three versions of bits/socket.h want to know whether __flexarr
will produce a real flexible array member -- specifically, one that
doesn't alter sizeof(the structure containing it).  They were testing
for this with a complicated #if condition that did not agree with
sys/cdefs.h and that tripped -Wundef warnings under -std=c90.
I added a new macro to sys/cdefs.h, __glibc_c99_flexarr_available,
which reveals exactly what these headers want to know.  I also took
the opportunity to flatten the rather messy conditional nest defining
__flexarr.

	* argp/argp.h: Check whether _LIBC is defined before expanding it.
	* posix/glob.h: Check whether __USE_XOPEN2K8 is defined instead
	of expanding it.

	* misc/sys/cdefs.h: Tidy up conditional nest defining __flexarr.
	Define __glibc_c99_flexarr_available to 1 when the compiler
	supports C99-compatible flexible array members, 0 otherwise.
	* sysdeps/unix/sysv/linux/bits/socket.h
	* sysdeps/mach/hurd/bits/socket.h
	* bits/socket.h: Use __glibc_c99_flexarr_available in
	definitions of struct cmsghdr and CMSG_DATA.

sysdeps/unix/sysv/linux/sys/timerfd.h

@@ -35,7 +35,7 @@ enum
 __BEGIN_DECLS
 
 /* Return file descriptor for new interval timer source.  */
-extern int timerfd_create (clockid_t __clock_id, int __flags) __THROW;
+extern int timerfd_create (__clockid_t __clock_id, int __flags) __THROW;
 
 /* Set next expiration time of interval timer source UFD to UTMR.  If
    FLAGS has the TFD_TIMER_ABSTIME flag set the timeout value is