mrpdclient: API change, un-globify the socket connection to mrpd

API change applied to mrpctl as the simplest sample code for it.
mrpdclient_init returns the socket:
- No need to have the socket global in mrpdclient lib, and allow its un-globification
  in applications.
- Applications can use the socket to build a poll/select event loop around it.
- Subsequent calls to mrpdclinet_* take the socket as additional parameter.
- Socket is closed and uninit in mrpdclient_close.

Note: this commit breaks the build of mrpq and mrpValidate.