Add support for CACHEDIR.TAG identification of cache directories.

Add support for CACHEDIR.TAG identification of cache directories.

Update include/exclude filtering with the ability to recognize and exclude
from backups any cache directories tagged according to
https://bford.info/cachedir/.

Configuration is described in more detail in doc/exclude.rst.  This is not
enabled by default, but the `--cachedir-check` option can be used to enable
it: this places a filtering rule that will recognize and exclude cache
directories.  Depending on where this rule is added compared to other
filtering rules, it is possible to force some cache directories to be
included.

When such a directory is excluded, the top-level CACHEDIR.TAG file in it is
still included in the backup, which should make it more obvious where
directories were automatically excluded.

The implementation could be structured better—logic is split between
exclude.cc and main.cc, and refactoring that code might be able to improve
things—but still shouldn't be too difficult to follow.