Make the gameplay affecting changes optional.
Merge https://github.com/chocolate-doom/chocolate-doom into crispy-doom
Merge branch 'master' into crispy-doom

Conflicts:
	src/m_config.c
New struct player2_t, port centermessage to player2_t.
Look up/down works.
Mouse look works.
Jumping works.
Fix sky stetching when free look or mouse look is enabled.
Apply some reasonable defaults for Crispy Doom.
Merge branch 'crispy-doom' into player2
Make jumping optional.
Retain demo compatibility, phew!
Generally suppress in-air movement when jumping is disabled.
Merge branch 'master' into crispy-doom
Allow both idspispopd and idclip in all gamemissions.
Use libsamplerate by default.
Suppress loading of internal or external DEH files or pack_nerve modifications if -nodeh is given.
Improve mouselook:

* Add a mousekey to temporarily enable mouselook
* Add separate mouse settings for vertical movement
* Apply novert only for translational movement
Add crosshair/laserpointer
Refine mouse look, suppress crosshair with secret message.