sig_pri.c: Pull the pri_dchannel() PRI_EVENT_RING case into its own function.

* Populate the CALLERID(ani2) value (and the special CALLINGANI2 channel
variable) with the ANI2 value in addition to the PRI specific ANI2 channel
variable.

* Made complete snapshot staging with the channel lock held.  All channel
snapshots need to be done while the channel lock is held.
........

Merged revisions 414050 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@414051 f38db490-d61c-443f-a65b-d21fe96a405b