Add stub setresuid(), setresgid()

Add to libc/user.cc stub setresgid(), setresgid(), which as usual in
OSv only allow user and group 0. These functions are used by Rogue ;-)

Also move getuid() and friends from runtime.cc to libc/user.cc, where
they feel more at home.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>