fusefs: attempt to support servers as old as protocol 7.4

Previously we allowed servers as old as 7.1 to connect (there never was a
7.0). However, we wrongly assumed a few things about protocols older than
7.8. This commit attempts to support servers as old as 7.4 but no older. I
added no new tests because I'm not sure there actually _are_ any servers
this old in the wild.

Sponsored by: The FreeBSD Foundation