Declare cuserid in unistd.h for UNIX98 and before (bug 20043).

For UNIX98 and older X/Open standards, unistd.h should have a
declaration of the legacy cuserid function, but such a declaration is
missing.  This patch adds that missing declaration.

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

	[BZ #20043]
	* posix/unistd.h [__USE_XOPEN && !__USE_XOPEN2K] (cuserid): New
	prototype.