Make the non-standard but extremely useful macro VA_ARG_COUNT()
available outside of dhcp.h.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

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

The home-grown versions don't work properly for static variadic
functions, when gcc can choose to use a non-standard calling convention.