fusefs: implement VOP_ACCESS

VOP_ACCESS was never fully implemented in fusefs. This change:

• Removes the FACCESS_DO_ACCESS flag, which pretty much disabled the whole vop.
• Removes a quixotic special case for VEXEC on regular files. I don't know why that was in there.
• Removes another confusing special case for VADMIN.
• Removes the FACCESS_NOCHECKSPY flag. It seemed to be a performance optimization, but I'm unconvinced that it was a net positive.
• Updates test cases.

This change does NOT implement -o default_permissions. That will be handled
separately.

PR: 236291
Sponsored by: The FreeBSD Foundation