Fix problems ofthe ifconfig argument parser with "ifconfig <if> join".

Fix problems ofthe ifconfig argument parser with "ifconfig <if> join".
Due to the way the parsing works, you cannot have an option that
accepts strings as argument or no argument, without side effects: for
example "join <nwid>" could only join networks that did not have a
nwid identical to another ifconfig option, i.e. "join mtu" or "join
join" would not work.

Solve this by making join always require an nwid. Listing all the
configured nwids for auto-join is moved to the new option "joinlist".
Removing _all_ auto-join configuration is moved to "-joinlist".

deraadt@ likes it and ok phessler@ stsp@