libc: Move getpwuid to user.cc

Move getpwuid() and getpwuid_r() to libc/user.cc where OSv user is
defined.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
libc: Fix getpwuid_r() to use OSv user info

The "user.home" JVM system property, for example, uses getpwuid() so
make sure it's returning proper values.

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