work_queue_worker now listens for SIGCHLD correctly.

Added link_usleep_mask() that accepts a signal mask, unblocks any signal provided during the poll itself (it restores the old mask afterwards), and exits if EINTR is thrown.




git-svn-id: file:///afs/nd.edu/user37/ccl/svn/cctools/trunk@2695 a4d8336d-3463-0410-8bba-c098c45d37a8