Add FILE_LICENCE declarations to almost all files that make up the
various standard builds of gPXE.

Code paths that automatically allocate memory from the FBMS at 40:13
should also free it, if possible.

Freeing this memory will not be possible if either

  1. The FBMS has been modified since our allocation, or

  2. We have not been able to unhook one or more BIOS interrupt vectors.

Add method for hooking real-mode interrupt vectors.