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

I want to get to the point where any header in include/ reflects a
standard user-level header (e.g. a POSIX header), while everything that's
specific to gPXE lives in include/gpxe/.  Headers that reflect a Linux
header (e.g. if_ether.h) should also be in include/gpxe/, with the same
name as the Linux header and, preferably, the same names used for the
definitions.

kernel.

Removed obsolete struct arprequest from if_arp.h and put it in nic.c so
that nic.c will still compile.  ARP will very shortly be handled by
net/arp.c instead.