fusefs: raise protocol level to 7.10

Protocol version 7.10 has only one new feature, and I'm choosing not to
implement it, so this commit is basically a noop. The sole new feature is
the FOPEN_NONSEEKABLE flag, which a fuse file system can return to indicate
that a certain file handle cannot be seeked. However, I'm unaware of any
file system in ports that uses this flag.

Sponsored by: The FreeBSD Foundation