Fix linknamespace h_errno handling.

Fix linknamespace h_errno handling.

Some linknamespace tests were failing because of references to
h_errno.  References to this symbol should be allowed for XPG4, UNIX98
and XOPEN2K; this patch adds it to the list of allowed data symbols
for those standards.  (h_errno was removed in the 2008 edition of
POSIX, so POSIX2008 and XOPEN2K8 references to it are genuine bugs;
fortunately it's not a public data symbol - __h_errno_location is
public, TLS h_errno is GLIBC_PRIVATE - so the symbol can simply be
renamed to __h_errno to fix those bugs without the ABI complications
normally associated with namespace issues for data symbols.)

Tested for x86_64.

	* conform/list-header-symbols.pl (%extra_syms): Add h_errno for
	XPG4, UNIX98 and XOPEN2K.
	* conform/Makefile (test-xfail-XOPEN2K/grp.h/linknamespace):
	Remove variable.
	(test-xfail-XOPEN2K/pwd.h/linknamespace): Likewise.