Fix vsyslog namespace (bug 18533).

Fix vsyslog namespace (bug 18533).

syslog functions (in POSIX) bring in the strong symbol vsyslog (not in
POSIX).  This patch fixes this by changing this symbol from a strong
alias to a weak alias.

Tested for x86_64 and x86 (testsuite, and that installed stripped
shared libraries are unchanged by the patch).  (vsyslog becomes weak
in the static libraries, which is what's needed; the particular macro
sequence in use leaves it as strong in the shared libraries, hence
those libraries being completely unchanged, but it doesn't generally
matter whether symbols exported from the shared libraries are weak or
strong.)

	[BZ #18533]
	* misc/syslog.c (vsyslog): Define as a weak alias of __vsyslog,
	not a strong alias.
	* conform/Makefile (test-xfail-XOPEN2K8/syslog.h/linknamespace):
	Remove variable.