Add bup prune-older command

Add bup prune-older command

prune-older removes (permanently deletes) all saves except those
preserved by various temporal keep arguments.  It is equivalent to a
suitable "bup rm" invocation followed by "bup gc".

For example, this invocation keeps all saves on the BRANCH for the past
month, and any older monthlies for the past year, and deletes the
remainder:

  $ bup prune-older --keep-all-for 1m --keep-monthlies-for 1y BRANCH

Tested-by: Rob Browning <rlb@defaultvalue.org>
Signed-off-by: Rob Browning <rlb@defaultvalue.org>