chan_pjsip: Add PJSIPHangup dialplan app and manager action

chan_pjsip: Add PJSIPHangup dialplan app and manager action

See UserNote below.

Exposed the existing Hangup AMI action in manager.c so we can use
all of it's channel search and AMI protocol handling without
duplicating that code in dialplan_functions.c.

Added a lookup function to res_pjsip.c that takes in the
string represenation of the pjsip_status_code enum and returns
the actual status code.  I.E.  ast_sip_str2rc("DECLINE") returns
603.  This allows the caller to specify PJSIPHangup(decline) in
the dialplan, just like Hangup(call_rejected).

Also extracted the XML documentation to its own file since it was
almost as large as the code itself.

UserNote: A new dialplan app PJSIPHangup and AMI action allows you
to hang up an unanswered incoming PJSIP call with a specific SIP
response code in the 400 -> 699 range.