Implemented tas_jb and tas_dtap, some minor refactoring

Also fixed a typo in client.cpp that prevents tas_dtap from stopping.  The
refactoring is to make sure when calling PM_PlayerMove ahead of time (done by
PM_JumpBug, for example) messages such as "dt" will not be printed.  The
printing of blocking data is implemented.

sv_db4c_ceil is added, though it is not implemented yet.
Clear data (tas_dtap etc) when client is connected

Data such as tas_dtap, tas_cjmp, tas_jb are cleared each time the client
connects to the server.  This is to ensure that, if, for example, the player
hasn't jumped for the designated number of times, then when we load a map or
from a saved state the player will not just start jumping.
Added HUD indication when blocked by certain planes

A yellow box will be displayed at the centre of the screen when the player is
blocked by planes with z component less than 0.7, which are considered to be
"ground" by PM_CatagorizePosition.  This box can be annoying (but helpful in
production), therefore hud_blocked, which controls its display, is 0 by
default.

It is probably unnecessary to use exp function to control its size, but that's
the first function that came to mind.  Also, the address stored in "blockaddr"
key is a 32-bit integer, which can cause trouble if valve ever releases a
64-bit Half-Life build.

Currently, things like positions and colours are hardcoded, which is
undesirable in the longrun.  We might add some cvars in the future to allow
customisations.
Made trigger_teleport vary its intensity greater

trigger_teleport has the same "status" as trigger_changelevel, so they should
be similarly conspicuous.
Changed the type of planes displayed when blocked

We find that, from experience and through some testings, displaying all but
perfectly horizontal planes might be more useful.  For example, we will then be
able to know if a slopejump is made when colliding with a slanted plane.