Arcyd: move signal handler setup to arcyd start

To better ensure graceful exit, register the sigterm handler at the
start of Arcyd's start routing instead of during abdi_processrepos.

Note that arcyd instaweb doesn't seem to exit if it's processing when
sigterm is received - something is catching the exception.  If it wasn't
for that then we'd probably want to cover everything from Arcyd's main()
routine.

Test Plan:
$ ./precommit.sh
Arcyd: BREAK, daemonize 'arcyd start' by default

Users will generally want Arcyd to be a long-running process that isn't
tied to any particular terminal or session.  To that end, make
'arcyd start' launch as a daemon by default, with stdout and stderr
redirected to ./var/log in the arcyd dir.

Provide a '--foreground' option for preventing the daemonization, useful
during tests and if a third-party solution for daemonization is to be
used instead.

Test Plan:

Update existing tests to make use of new functionality.

$ ./precommit.sh
$ ./testbed/arcyd/test_shell.sh
$ ./testbed/arcyd/exercise_arcyd.sh
$ ./testbed/arcyd/acceptance_tests.sh