Add FILE_LICENCE declarations to almost all files that make up the various standard builds of gPXE.
Achieved via Perl using: perl -pi -e 's/pk_buff/io_buffer/g; s/Packet buffer/I\/O buffer/ig; ' \ -e 's/pkbuff\.h/iobuf.h/g; s/pkb_/iob_/g; s/_pkb/_iob/g; ' \ -e 's/pkb/iobuf/g; s/PKB/IOB/g;'