Fix externalivr's setvariable command so that it properly sets multiple variables.

Fix externalivr's setvariable command so that it properly sets multiple variables.

The command had a for loop that was guaranteed to only execute once since
the continuation operation of the loop would set the input buffer NULL. I rewrote
the loop so that its operation was more obvious, and it would set multiple variables
correctly.

I also reduced stack space required for the function, constified the input string,
and modified the function so that it would not modify the input string while I was
at it.

(closes issue #15114)
Reported by: chris-mac
Patches:
      15114.patch uploaded by mmichelson (license 60)
Tested by: chris-mac

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@195316 65c4cc65-6c06-0410-ace0-fbb531ad65f3