Add lockfile for control of UserCouch.basedir

[Jason Gerard DeRose]

CouchDB has a rather unfortunate behavior in that multiple instances will happily start using the exact same database and view files. As CouchDB doesn't do any kind of sane locking, UserCouch really should.

This will also help making the development process nicer. As multiple processes can bind to org.freedesktop.Dmedia, often you'll mistakenly have both the system-wide Dmedia running and dmedia-service running from inside the source tree. Some simple locking will help prevent possible screwy behavior.

So I'm adding a simple lock using fcntl.flock(). Although not portable, it's reliable and easy to work with.

Something still to work out is how this lock is going to behave with regard to beam not actually terminating when the user logs out. desktopcouch had this problem too (and AFAIK, they never came up with a solution).