Add REPLACE & PASSTHRU functions, overhaul of func_strings, fix API docs for...

Add REPLACE & PASSTHRU functions, overhaul of func_strings, fix API docs for the ast_get_encoded_* functions.
 * Add REPLACE function, which searches a given variable for a set of
   characters and replaces each with a given character.
 * Add PASSTHRU function, which passes a literal string back, like a NoOp for
   functions.  Intent is to be able to specify a literal string to another
   function that takes a variable name as an argument.
 * Let the array manipulation functions work with dialplan functions, in
   addition to variables.  This allows the array manipulation functions to
   modify ASTDB and ODBC backends, assuming the func_odbc configuration has
   both read and write functions.
(closes issue #15223)
 Reported by: ajohnson
Patches: 
       20091112__issue15223.diff.txt uploaded by tilghman (license 14)
 Tested by: lmadsen, tilghman


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@230994 65c4cc65-6c06-0410-ace0-fbb531ad65f3