fuse-cmd.py: given --meta, report original metadata

Add a new --meta option to "bup fuse", and when specified, report the
original mode, uid, gid, atime, mtime, and ctime for the mounted
paths.

Since negative timestamps cause access errors, and cause "ls -l" to
report question marks for the stat values, set a floor of 0 for all
timestamps reported to python-fuse.

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