Add MatchEntry::parse_pattern

Add MatchEntry::parse_pattern

This is a convenience helper to parse an entry with possible
inversion via a leading exclamation mark and interpreting
trailing slashes to mean `MatchFlag::MATCH_DIRECTORY` and
create a `Pattern` from the string with those characters
stripped off.

This also changes the way `MatchFlag`s are passed to a
builder pattern for convenience.

Also: impl std::ops::Not for MatchType.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>