And update tests as well.

With the ~--eval~ option you can evaluate an expression in mu/scm
environment. For example:

$ mu scm --eval \
  '(format #t "found ~d match(es)\n" (length (mfind "hello")))'
found 7173 match(es)

Add command-line parameter and implement.

Esp., better handle the 'no subcommand' case: give the user some more useful
help.

- don't make assumptions on where programs live (i.e., /bin/sh, /bin/rm,
/bin/mv) are not universal
- dont invoke shell when unnecessary
- improve error-handling

When `--quiet` is passed (and not --debug), do not log with g_message at
startup.

Update names, docs etc.

Split labels-cache and store-labels.

E.g. with unexpected termination for mu it is possible that the
labels-cache (i.e., the one that is used for auto-completion) gets disconnected
from reality.

Add a --restore option to `mu label list` to restore the actual labels from the
labels seen in the store.