Adjust metadata handling for python 3

Adjust metadata handling for python 3

Adapt bup.metadata for python 3 and the other bits directly affected:
bup-ftp, bup-ls, bup-meta, bup-xstat, and bup.ls.

Rename metadata detailed_str() and summary_str() to detailed_bytes()
and summary_bytes() since they aren't (and absolutely should not be)
localized.  They produce output that should be suitable for
programmatic use, i.e. "bup ls | grep ...".  Not sure we'll keep those
names, but they'll do for now.

Also rename fstime_to_sec_str() to fstime_to_sec_bytes() since that's
the only way we ever use it.

Make a minimal change to bup-ftp for now -- just enough to handle the
changed ls.within_repo arguments.

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