lockd.c: Support the --keep-running argument

This argument prevents the daemon from exiting after the last client exits.
Saves initialization time if a database is going to be accessed many times.

Signed-off-by: Morten Hustveit <morten.hustveit@gmail.com>
Avoid hard-coding path and backend of ca_catalog.ca_columns table

Because the ca_catalog.ca_columns table is a write-once table, its path changes
whenever its contents changes.

Signed-off-by: Morten Hustveit <morten.hustveit@gmail.com>