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

equivalents for alloc_pkb() and free_pkb(), which will be retired in
due course).

is now gPXE-specific.  (The standard malloc() et al have been in stdlib.h
for a while).

Add free memory counter.

hardware that can't autopad).

crossings.

implementation allows for only one, and does so without compromising on
the efficiency of static allocation).

Link-layer protocols are cleanly separated from the device drivers.

Network-layer protocols are cleanly separated from individual network
devices.

Link-layer and network-layer protocols are cleanly separated from each
other.