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

The DHCP client code now implements only the mechanism of the DHCP and
PXE Boot Server protocols.  Boot Server Discovery can be initiated
manually using the "pxebs" command.  The menuing code is separated out
into a user-level function on a par with boot_root_path(), and is
entered in preference to a normal filename boot if the DHCP vendor
class is "PXEClient" and the PXE boot menu option exists.

screen sequence doesn't work (like bios_console.c on some platforms).

the (slow) werase().

- no longer depends on slk

library use later on
- some small mods to existing functions