always-mouselook cvar

Resolves issue #55.

This adds the archived cvar m_look, exposed in the menu as "always mouselook".

m_look 1 is the default. When m_look is 1, vertical mouselook is always
enabled, and the +mlook state is irrelevant.

m_look 0 behaves like original Quake, where +mlook must be active in order to
have vertical mouselook.