Make the POSIX::setuid and POSIX::setgid to really call setuid()

Make the POSIX::setuid and POSIX::setgid to really call setuid()
and setgid() because they were just changing $< and $( which means
only changing the real uid/gid, as opposed to changing both
real and effective ids.  (The alternative way could have been
in POSIX.pm to change $> and $), too, but making a direct call
to the C API feels cleaner.)  Fixes the bug

Subject: [ID 20000904.005] POSIX::setuid() Doesn't Call setuid()
Message-Id: <200009041609.e84G9iN12155@ifr.inside.zvolve.net>

p4raw-id: //depot/perl@7521