fusefs: fix intermittency in the interrupt tests

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