Use a system-supplied maximum number of signals on Mac OS X.  This gets SIGUSR2 (#31) working.
Merge pull request #85 from r1mikey/master

Use a system-supplied maximum number of signals on Mac OS X.
Avoid double-decrement of level->todo

If a job's dispatch function deletes the job, we may end up decrementing
the job queue's todo count twice. Verify that the job is on the queue
before decrementing.
Detect the max signal value that can be used using NSIG macro
Merge pull request #87 from anchor/del-during-dispatch-v2

Avoid double-decrement of level->todo