Blacklist possibly-bad syscalls during diversions, instead of whitelisti...

...ng known-to-be OK ones.

This turned out to be particularly irritating because it turned out that the call_function test wasn't actually running.  (Luckily it hadn't regressed.)  And then for some reason, with nonzero return from clock_gettime to the call_function test, gdb all of a sudden started trying to write xsave registers.  That's stubbed out for now because we're about to add that support for real.