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.