Rename "rpm" command to "db"

Eliminates some confusion between "rpm-ostree rpm" (or "atomic rpm")
commands versus actual "rpm" commands.

The "rpm" subcommand is retained as a hidden alias for the "db"
subcommand for backward-compatibility.  It is not listed in --help
output.

Fixes #22
Split up rpmostree-builtin-db.c

As a followup to renaming the "rpm" command to "db", split the "db"
subcommands into separate source files in the style of "ostree admin"
and "rpm-ostree compose".

Also create rpmostree-rpm-util.[ch] as a place for common rpm-related
functions needed by the "db" subcommands.

No intentional functional changes here, just a bunch of copy-n-paste
and minor cleanup.