Implement --signature-filter for signing backup snapshots.

Implement --signature-filter for signing backup snapshots.

If supplied, the argument to --signature-filter will be a program through
which the text for the root snapshot descriptor is filtered.  This filter
should act much like "gpg --clearsign": produce a nearly-identical text
file, with perhaps a few lines at the start or end containing the
signature, but which can be treated as an ordinary segment descriptor by
ignoring this leading and trailing data.