fusefs: fix intermittency in the interrupt tests

• In the fatal_signal test, wait for the daemon to receive FUSE_INTERRUPT before exiting.
• Explicitly disable restarting syscalls after SIGUSR2. This fixes intermittency in the priority test. I don't know why, but sometimes that test's mkdir would be restarted, and sometimes it would return EINTR. ERESTART should be the default.
• Remove a useless copy/pasted sleep in the priority test.

Sponsored by: The FreeBSD Foundation