fusefs: raise protocol level to 7.15

This protocol level adds two new features: the ability for the server to
store or retrieve data into/from the client's cache. But the messages
aren't defined soundly since they identify the file only by its inode,
without the generation number. So it's possible for them to modify the
wrong file's cache. Also, I don't know of any file systems in ports that
use these messages. So I'm not implementing them. I did add a (disabled)
test for the store message, however.

Sponsored by: The FreeBSD Foundation