Add a Quake Live-style mouse acceleration mode.

Add a Quake Live-style mouse acceleration mode.

Its cvars are m_accel_power*, and the cvar meanings are as close as possible
to the ezQuake implementation (as its source is available). Names differ though
due to conflicts with the current linear acceleration mode.

Original code by Frank <franparpe@franparpe.org>, cleanups by Rudolf Polzer <divVerent@xonotic.org>.

From: Frank <franparpe@franparpe.org>

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@12523 d7cf8633-e32d-0410-b094-e92efae38249