Implement AMI action PRIShowSpans.

PRIShowSpans works like the AMI action DAHDIShowChannels but for PRI
spans.  It is similar to the CLI command "pri show spans".

(closes issue #15980)
Reported by: dwery


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@314735 65c4cc65-6c06-0410-ace0-fbb531ad65f3