Validate command and cvar names
Avoid multiple hash lookups in cmd/cvar/alias unordererd_map
Fix Alt+Enter not toggling fullscreen

The cvar was CVAR_LATCH so the value wasn't updated.
It didn't need to be latch because sdl_glimp.c keeps the current fullscreen
state in a separate variable.
Update NaCl SDK paths
Allow command names starting with +/-
Merge branch 'engine-upgrade/master' into engine-upgrade/for-merge