Adds STRREPLACE function

Adds a new STRREPLACe function to func_strings.c that allows users to search and replace
against a variable in the dialplan.

(closes issue #18023)
Reported by: wdoekes

Review: https://reviewboard.asterisk.org/r/1219/ 


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@320040 65c4cc65-6c06-0410-ace0-fbb531ad65f3