chan_dahdi: CLI "core stop gracefully" has needless delay for PRI and SS7.

The PRI and SS7 link control threads are not stopped correctly when the
chan_dahdi.so module is unloaded.  The link control threads pri_dchannel()
and ss7_linkset() are not awakened from a poll() to cancel the thread.

* Added a SIGURG signal after requesting the thread cancel to break the
link control thread poll() immediately.

For SS7 it was slightly worse, the link poll() timeout would always be
whatever was the last libss7 scheduled event time used.  If no libss7
scheduled event was pending, the thread could run more often than
necessary.

* Set nextms to 60 seconds for the ss7_linkset() poll() if there is no
other libss7 scheduled event.


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@399818 f38db490-d61c-443f-a65b-d21fe96a405b