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

Conventional usage of the various struct sockaddr_xxx types involves
liberal use of casting, which tends to trigger strict-aliasing
warnings from gcc.  Avoid these now and in future by marking all the
relevant types with __attribute__((may_alias)).

Remove various 32-bit assumptions scattered throughout the codebase.
The code is still not necessarily 64-bit clean, but will at least
compile.

functions.