Make accel() method accessible from Squirrel scripts.

The function is really a member of WarField, but defined in CoordSys
for particular reason.
Add slen() member function to Vec3d in Squirrel scripts.

Also enabled debugging information for debug builds.
surface: Make Hellfire missile's homing calculation scripted.

Now we can experiment and tweak the algorithm with the game being kept
running, just like Aerial's case.

The algorithm is just dead copied from C++ source, plus some dynamic
variables per Hellfire instance.  The script provided per-instance
variables are stored in tables referred to with the name "object".