Master Server: Added a 'locked' property for future use

A locked server will require the user to supply a password in order
to join the game session as a player. Note that the password itself
is NOT announced to master, only the existence of it.
Homepage|Master Browser|Fixed: Misinterpretation of a server's 'open' status

Rather than denoting the existence of a password to join the game
session, the 'open' status instead refers to whether the server has
a full compliment of players.

Now uses the newly added 'locked' property for this purpose.
Merge branch 'master' of github.com:skyjake/Doomsday-Engine