Add a test for page fault handling.

This both tests that they work correctly and prints timing information
if you run it manually.
Add a define to use sigaction on OS X instead of OS-specific page fault handling.

The latter was introduced last year in order to allow debugging without
turning off Fastmem; I wanted to see if the old way was faster.  In
fact, it is not, but may as well keep the define around in case future
testing is necessary.