Enhancement: Reject files/dirs by name first

The current scanner/archiver code had an architectural limitation: it always
ran the `lstat()` system call on all files and directories before a decision to
include/exclude the file/dir was made. This lead to a lot of unnecessary system
calls for items that could have been rejected by their name or path only.

We've changed the archiver/scanner implementation so that it now first rejects
by name/path, and only runs the system call on the remaining items. This
reduces the number of `lstat()` system calls a lot (depending on the exclude
settings).

https://github.com/restic/restic/issues/1909
https://github.com/restic/restic/pull/1912