Add support for passing credentials over Unix sock

Add support for passing credentials over Unix sock

Add lttcomm_* functions for sending, receiving and setting socket option
to pass credentials over a Unix socket. Those functions allowed normal
data to be pass as well.

First commit of a series to enable credentials passing between the lttng
command line client and the session daemon.

Signed-off-by: David Goulet <david.goulet@polymtl.ca>