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

There are many functions that take ownership of the I/O buffer they
are passed as a parameter.  The caller should not retain a pointer to
the I/O buffer.  Use iob_disown() to automatically nullify the
caller's pointer, e.g.:

    xfer_deliver_iob ( xfer, iob_disown ( iobuf ) );

This will ensure that iobuf is set to NULL for any code after the call
to xfer_deliver_iob().

iob_disown() is currently used only in places where it simplifies the
code, by avoiding an extra line explicitly setting the I/O buffer
pointer to NULL.  It should ideally be used with each call to any
function that takes ownership of an I/O buffer.  (The SSA
optimisations will ensure that use of iob_disown() gets optimised away
in cases where the caller makes no further use of the I/O buffer
pointer anyway.)

If gcc ever introduces an __attribute__((free)), indicating that use
of a function argument after a function call should generate a
warning, then we should use this to identify all applicable function
call sites, and add iob_disown() as necessary.

These cards very nearly support our current IB Verbs model.  There is
one minor difference: multicast packets will always be delivered by
the hardware to QP0, so the driver has to redirect them to the
appropriate QP.  This means that QP owners may see receive completions
for buffers that they never posted.  Nothing in our current codebase
will break because of this.

the assert message's file and line number gives some clue as to the
real location of the problem.

Added similar assertions to list.h.

the assert message's file and line number gives some clue as to the
real location of the problem.

Added similar assertions to list.h.

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