Implement new configuration option "DefaultUserModes"

The new configuration option "DefaultUserModes" lists user modes that
become automatically set on new local clients right after login.

Please note that only modes can be set that the client could set on
itself, you can't set "a" (away) or "o" (IRC Op), for example! User
modes "i" (invisible) or "x" (cloaked) etc. are "interesting", though.

Default: set no modes (like without this patch).

Closes bug #160.