PICARD-437: Picard can have problems saving files to network shares on Windows 8(?) when embedding cover art

Wrote this a while ago and forgot to submit it.

The functions _insert_bytes_no_mmap and _delete_bytes_no_mmap call mutagen.util.lock, which in turn uses the fnctl module which isn't available on Windows.

This adds a new method to lock things correctly. It's hacking around mutagen internals, so perhaps this is better fixed within mutagen itself (but I'm not sure what the likelihood of that is).

voiceinsideyou says the patch fixes his problem, and to date I think he's the only one to report it happen.

See http://tickets.musicbrainz.org/browse/PICARD-437