set_protocol_handler: new util

set_protocol_handler: new util

This sets the protocol handler of a given protocol to either a local
program or a web service.  Its first argument is the protocol given as a
string.  The second argument can be null, an nsIFile, or a string.  Null
cancels any existing handler and will cause it to prompt next time.  An
nsIFile gives an external program to run as a handler with the url as its
argument.  Generally use 'make_file' or 'get_file_in_path' for this.  The
argument can also be a string that gives an url template for a web
service.  The sequence "%s" within this string will be substituted for the
url-encoded url.