main: Delete lockfile when signal arrive too early

Previously, if signal was sent in between lock file creation and
signal handler registration, booth daemon exited without deleting lock
file.

I was able to reproduce this behaviour very rarely (and only
on s390 architecture) when running test suite. Reliable reproducer was
to put sleep call after call of create_lockfile and before call of
signal function,

Proposed solution is to move signal and atexit call before
create_lockfile.

Signed-off-by: Jan Friesse <jfriesse@redhat.com>