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

F8 is represented by the ANSI escape sequence "^[[19~", which is not
representable as a KEY_xxx constant using the current encoding scheme.
Adapt the encoding scheme to allow F8 to be represented, since PXE
requires that we may need to prompt the user to press F8.

Signed-off-by: Alexey Zaytsev <alexey.zaytsev@gmail.com>