lib: add support for path: prefix searches

lib: add support for path: prefix searches

The path: prefix is a literal boolean prefix matching the paths,
relative from the maildir root, of the message files.

path:foo matches all message files in foo (but not in foo/new or
foo/cur).

path:foo/new matches all message files in foo/new.

path:"" matches all message files in the top level maildir.

path:foo/** matches all message files in foo and recursively in all
subdirectories of foo.

path:** matches all message files recursively, i.e. all messages.