Show plane angle on HUD, prevent text colour change

With hud_planeang_z 1 the arc cosine (in degrees) of the z component of the
plane's normal is displayed on the HUD.  If the colour of the text is green,
that means the player can stand on the plane.

In addition, the colour of the first letter "H" is now always yellow.
Previously, if some text is displayed on the screen (e.g. chapter titles) then
the colour of the "H" letter may change.
Fix HUD yellow box size across level transition

Occasionally, flTime can become less than prevBlockTime, thus making the size
of the yellow box extremely large.  This can happen when there is a map change.
Implemented tas_db4c
Implemented tas_db4l
Avoid unnecessary ducking if db4c or db4l failed
Avoid double jump sound when landing if db4l is on

We did not care about tfc jump sound in the wrapper function as this mod is not
meant for tfc.
Clear tas_db4c and tas_db4l upon client connect