Split game start sequence into two-stage process. This is the first
stage in refactoring the way that network startup works.

Subversion-branch: /branches/v2-branch
Subversion-revision: 2580
Don't start the game until all players are ready. Send waiting data to
players that are ready for startup progress screens.

Subversion-branch: /branches/v2-branch
Subversion-revision: 2581
Split D_InitNetGame() into two separate functions for startup.

Subversion-branch: /branches/v2-branch
Subversion-revision: 2582
Split Doom netgame startup into two stages.

Subversion-branch: /branches/v2-branch
Subversion-revision: 2583
Use two-stage netgame startup for Heretic.

Subversion-branch: /branches/v2-branch
Subversion-revision: 2584
Use two-stage startup for Hexen, and add netgame startup callback for
the spinal loading screen.

Subversion-branch: /branches/v2-branch
Subversion-revision: 2585
Use two-stage netgame startup for Strife, and restore the previous
startup splash-screen behavior (before it was modified in r2435).

Subversion-branch: /branches/v2-branch
Subversion-revision: 2586