relayd: clean-up: reduce the number of exit paths in main()

The relay daemons's initialization and teardown relies on a large
number of goto/labels to clean-up on failure to launch.

All of the clean-up can be performed unconditionally by checking for
NULL or uninitialized file descriptors in relayd_cleanup(). This
reduces the number of possible tear-down paths and partly initialized
state to keep track of.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I3195caf05b3dd04cc7e9c60dcb322261159fe0df