[Asterisk-Dev] Major dialplan functions rework

[Kevin P. Fleming]

See the commit just posted to asterisk-cvs... I made some significant 
changes in preparation for the bevy of new functions that are coming as 
people start converting existing applications to functions.

There are some non-backwards-compatible changes here:

- 'struct ast_custom_function_obj' became 'struct ast_custom_function' 
to match the other API structs (ast_cli_entry, etc.)
- the regex, isnull, if and exists functions changed to REGEX, ISNULL, 
IF and EXISTS to match the new naming policy
- functions are now to be stored in source files in the funcs 
subdirectory, instead of in pbx.c; see the Makefile in that directory if 
you are interested in how they all end up in pbx_functions.so

If you have out-of-tree functions, I'm sorry for the little bit of extra 
work to update (just a search and replace), but hopefully the ability to 
drop them into the funcs subdirectory and have them built automatically 
will ease your pain :-)