Fix nice getpriority, setpriority namespace (bug 18553).

nice (XPG3) calls getpriority and setpriority (in XPG4 but not XPG3,
i.e. UX-shaded in XPG4).  This patch fixes this by making those
functions into weak aliases of __* functions and calling the __*
versions as needed.

Tested for x86_64 and x86 (testsuite, and that disassembly of
installed shared libraries is unchanged by this patch).

This completes cleaning up the unsorted linknamespace test XFAILs.

	[BZ #18553]
	* resource/getpriority.c (getpriority): Rename to __getpriority
	and define as weak alias of __getpriority.
	* resource/setpriority.c (setpriority): Rename to __setpriority
	and define as weak alias of __setpriority.
	* sysdeps/mach/hurd/getpriority.c (getpriority): Rename to
	__getpriority and define as weak alias of __getpriority.
	* sysdeps/mach/hurd/setpriority.c (setpriority): Rename to
	__setpriority and define as weak alias of __setpriority.
	* sysdeps/unix/syscalls.list (getpriority): Use __getpriority as
	strong name.
	(setpriority): Use __setpriority as strong name.
	* sysdeps/unix/sysv/linux/getpriority.c (getpriority): Rename to
	__getpriority and define as weak alias of __getpriority.
	* include/sys/resource.h (__getpriority): Declare.  Use
	libc_hidden_proto.
	(__setpriority): Likewise.
	(getpriority): Don't use libc_hidden_proto.
	(setpriority): Likewise.
	* sysdeps/posix/nice.c (nice): Call __getpriority instead of
	getpriority.  Call __setpriority instead of setpriority.
	* conform/Makefile (test-xfail-XPG3/unistd.h/linknamespace):
	Remove variable.