- construct the difference and have only one call to snprintf in build_via

- convert some while loops to for loops
- localize some variables and remove unneeded initializations
- store result of get_header locally so it only has to be called once
- remove some duplicate calls to strcasecmp by storing result
(issue #5955)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@7587 65c4cc65-6c06-0410-ace0-fbb531ad65f3