Fix gman_servers_* functions

Add a few new functions to manage Gearman server lists. This will probably be: gman_servers_set, gman_servers_add, gman_servers_del, gman_servers_list. Each (except list) will take a command separated list of server[:port] and an optional function name.

On second though, server_set() with proper behavior is what most folks needs (replace list, not add). Marking this as complete since set() behavior is now changed. If someone really wants these other functions, we can make this active again.