add option 'a' to chanisavail.

If you give chanisavail a list of channels, it will only
return the first available channel.
When this option is set, it will return all the available
channels from the given list.

(closes issue #12248)
Reported by: dagmoller
Patches:
      app_chanisavail-snv.patch-v2.txt uploaded by dagmoller (license 436)
	   - major changes by me because russellb pointed out some buffer overflows
	     and codeguideline issues.
		 Converted it all to the ast_str_* api
Tested by: dagmoller, mvanbaak


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@118101 65c4cc65-6c06-0410-ace0-fbb531ad65f3