Make PurplePluginProtocolInfo definitions consistent

Since we can't use C99 structure initialisers, we have to manually add
new NULL fields to all protocols whenever we extend the structure.

Make it slightly easier to script that, by making the current last
field (get_cb_alias) consistent in all cases. In particular, there's
no reason *not* to have the trailing comma, as most already do.

Now I can add a new field to the PRPL by doing something like this...

PROTOFILES=`grep -rl '[A-Za-z_][A-Za-z0-9_]*,[[:space:]]*/\* get_cb_alias \*/' libpurple/protocols/ `
sed '/\/\* get_cb_alias \*\//{p;s/[A-Za-z_][A-Za-