Fix fgets_unlocked namespace issues (bug 17664).

Fix fgets_unlocked namespace issues (bug 17664).

Various POSIX functions bring in res_init.o, res_hconf.o or
mntent_r.o, which use fgets_unlocked, which is not a POSIX function.
This patch arranges for them to use __fgets_unlocked instead.  (The
IS_IN (libc) conditional in rec_hconf.c is needed because that file is
also used in nscd.)

Tested for x86_64 (testsuite, and that disassembly of installed shared
libraries is unchanged by the patch except for an assertion line
number).  Note that most of the linknamespace tests that failed
because of fgets_unlocked from the resolver also fail because of other
symbols brought in by the resolver, so the number of XFAILs this
removes is limited.  Also note that fgets_unlocked failures for
unistd.h for XPG3/XPG4 showed up that actually unistd.h is declaring
too much for XPG3/XPG4 (bug 17665) - there is no actual need to make
getusershell.c use __fgets_unlocked (at least as regards formal
standards are concerned; maybe it should still change for
namespace-cleanness of _DEFAULT_SOURCE) because the functions there
aren't actually in any of the supported standards; the correct fix for
those failures will be to stop the *usershell* functions appearing in
unistd.h for XPG3/XPG4.

	[BZ #17664]
	* misc/mntent_r.c (__getmntent_r): Use __fgets_unlocked instead of
	fgets_unlocked.
	* resolv/res_hconf.c [IS_IN (libc)] (fgets_unlocked): Define to
	__fgets_unlocked.
	* resolv/res_init.c (__res_vinit): Use __fgets_unlocked instead of
	fgets_unlocked.
	* conform/Makefile (test-xfail-XPG4/sys/statvfs.h/linknamespace):
	Remove variable.
	(test-xfail-POSIX/sys/mman.h/linknamespace): Likewise.
	(test-xfail-UNIX98/sys/mman.h/linknamespace): Likewise.
	(test-xfail-UNIX98/sys/statvfs.h/linknamespace): Likewise.
	(test-xfail-XOPEN2K/sys/mman.h/linknamespace): Likewise.
	(test-xfail-XOPEN2K/sys/statvfs.h/linknamespace): Likewise.
	(test-xfail-POSIX2008/sys/mman.h/linknamespace): Likewise.
	(test-xfail-POSIX2008/sys/statvfs.h/linknamespace): Likewise.
	(test-xfail-XOPEN2K8/sys/mman.h/linknamespace): Likewise.
	(test-xfail-XOPEN2K8/sys/statvfs.h/linknamespace): Likewise.