* walls are now filled, and displayed on top of aliens.

* fixed an overflow in the tunnel update code.
  * fragbomb is now 'f', not 'd'.
  * added a missing call to init_bonus().

* the starfield is now an array of stars.
  * fixed wall rendering (offset issues and disappearing left wall).
  * 32 seekers on fragbomb explosion, not 16.

* 16 seekers on fragbomb explosion instead of 12.

* factorized the seeker generation on fragbomb explosion.

* new super weapon, the fragmentation bomb.

* added GPL headers to source files.

* beam now collides with aliens.

* two new types of aliens. only one rules.
  * removed gfx_write in favor of gfx_putchar and gfx_putstr.
  * added bonuses at alien death. they do nothing yet.
  * seeker missiles. 'b' to test.
  * weapon resolution is now 16*char. needs to be generalized.
  * fixed the supernova bugs. center could collide with the tunnel, and
    the last frame was badly displayed.
  * lots of cleanups everywhere.

* used a cool Bresenham algorithm to draw the nuke circles.
  * nuke collides with aliens.

the nuke is still ugly, almost no collision detection. Nice demo :)