fuse(4) use a global environment check.

This is marginally faster than using an environment check in each test case.
Also, if the global check fails then all of the tests are skipped. Oddly,
it's not possible to skip a test in any other way.

Also, allow the test to run as a normal user if vfs.usermount=1 and
/dev/fuse is accessible.

Reported by: ngie
Sponsored by: The FreeBSD Foundation