Fix syslog dprintf namespace (bug 18534).

Fix syslog dprintf namespace (bug 18534).

syslog functions bring in references to dprintf, which wasn't added to
POSIX until the 2008 edition and so isn't in various standards
containing the syslog functions.  This patch fixes this by making
dprintf into a weak alias of __dprintf and using __dprintf as
appropriate.

Tested for x86_64 and x86 (testsuite, and that installed stripped
shared libraries are unchanged by the patch).

	[BZ #18534]
	* stdio-common/dprintf.c (__dprintf): Use libc_hidden_def.
	(dprintf): Define as a weak alias of __dprintf, not a strong
	alias.
	* include/stdio.h (__dprintf): Declare.  Use libc_hidden_proto.
	* misc/syslog.c (__vsyslog_chk): Call __dprintf instead of
	dprintf.
	* conform/Makefile (test-xfail-XPG4/syslog.h/linknamespace):
	Remove variable.
	(test-xfail-UNIX98/syslog.h/linknamespace): Likewise.
	(test-xfail-XOPEN2K/syslog.h/linknamespace): Likewise.