Core: switch user after Bind port.

When running in newer systems with Kernel >= 3.9 and the SO_REUSEPORT
option is used, the server was switching to the unpriviledge user before
to bind the TCP port, this was generating problems when the port was <= 1024.

Signed-off-by: Eduardo Silva <eduardo@monkey.io>
Core: fix user switch when running as root

Previous this patch the core was using real user id as reference
to perform user switch but when restoring back this is not allowed
by the Kernel, the right way to do it is through the effective user ID.

Signed-off-by: Eduardo Silva <eduardo@monkey.io>
Merge branch 'master' of github.com:monkey/monkey