fusefs: raise protocol level to 7.12

This commit raises the protocol level and adds backwards-compatibility code
to handle structure size changes. It doesn't implement any new features.
The new features added in protocol 7.12 are:

• server-side umask processing (which FreeBSD won't do)
• asynchronous inode and directory entry invalidation (which I'll do next)

Sponsored by: The FreeBSD Foundation