Save metadata during "bup save".

Save metadata during "bup save".

Record metadata in a hidden/mangled file named .bupm in each
directory, so that the metadata for /foo/ is stored in /foo/.bupm,
with the first entry being the metadata for foo/ itself.

Record an empty index file for each special file so that index entries
and .bupm entries correspond correctly.

Rework the strip/graft functions to return both the save-name, and the
underlying filesystem path to the save-name (when there is one) for
each component.  There may not be corresponding filesystem paths if
graft options rewrite the path prefix.

For now, record "/" metadata only when there are no strip/graft
options.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Reviewed-by: Zoran Zaric <zz@zoranzaric.de>
Tested-by: Alexander Barton <alex@barton.de>