Fix bug #185 - Args of !commands lost/damaged.
All string arguments were incorrectly freed after being added to the argument list for a command.
Instead this fix makes sure it is properly copied into a new string, and by doing so this requires
a new API for dealing with hub command arguments in a type safe manner, and also allows for each
argument to be cleaned up properly when the command is no longer needed.
This also fixes issues with parse errors for certain types, and optional arguments (previously it was impossible
to tell the difference for an integer with value 0 or if no integer was given).
All arguments can now be accessed through the new functions
hub_command_arg_reset() and hub_command_arg_next().
These functions are also exposed to plug-ins.
The argument type notations for 'n' has changed to mean nick (string),
and 'u' is used for a user (struct hub_user - must be online).
