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
Use sizeof to get the correct size of the sockaddr_un sun_path member in a portable way. Fixes corosync on Mac OS X.
Merge pull request #88 from r1mikey/master

Fix sockaddr_un.sun_path overflow on Mac OS X 10.9