This makes `metastore` more useful in scripts, just to check if it found
anything, in the same way as `diff` and `grep`.

Note that POSIX requires that EXIT_SUCCESS is 0, and EXIT_FAILURE is 1,
so I've defined EXIT_DIFFERENCES as 2 which, while different from the
values `diff` and `grep` use (1 for finding something, 2 for errors),
maintains a certain amount of backwards compatibility with previous
versions of metastore.

If you're storing metadata in a version control system with multiple
branches, mtime differences are going to produce a whole bunch of
conflicts that you likely don't care about. This allows you to not
save mtime and avoid those.

Note that we use a sentinel value of -1 for the mentry `mtimensec` field
to indicate this in the data, as all values of `mtime` are theoretically
valid, but `mtimensec` must always be between 0 and 999,999,999 in the
real world.

I'm not 100% sure about the mechanism for selecting this feature. The
legacy behaviour for metastore was to save mtimes in the metadata files,
but ignore them for compare/apply by default, with a `--mtime` option to
use the mtime data.

Keeping the legacy behaviour for backwards compatibility, but adding a
`--no-mtime` option to ignore mtimes when saving felt like a reasonable
way of making this happen, but something about it doesn't feel great.

Maybe I just didn't figure out how to make the documentation clear
enough.  ¯\_(ツ)_/¯

Only version 0 is supported here.

As a bonus you can install out-of-tree now via make -f.
Fixes #22.