major re-work of dialplan functions, including:

 - locking of functions list during registration/unregistration/searching
 - rename of function description structure to be consistent with the rest of the API
 - addition of 'desc' element to description structure, for detailed description (like applications)
 - addition of 'show function' CLI command to show function details
 - conversion of existing functions to use uppercase names to match policy
 - creation of new 'pbx_functions.so' module to contain standard 'builtin' functions
 - removal of all builtin functions from pbx.c and apps and placement into new 'funcs' directory


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@5583 65c4cc65-6c06-0410-ace0-fbb531ad65f3