Renamed .inc files to .php files

.inc extension for included PHP files is not recommended, instead
standard .php should be PHP files extension.
Convert tabs to spaces.
Splitted classes from xmlrpc.php to separate files
Renamed Xmlrpc class to Phpxmlrpc
Moved Phpxmlrpc class to own file
Modernized old classes
xmlrpcmsg: Convert createPayload() from private to public

Apparently this was used elsewhere, even when it was marked as private.
Adjusting to that now...