Check that the reference count is valid (i.e. non-negative) on each
call to ref_get() and ref_put(), using an assert() at the point of
use.

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

Since more reference-counted structures than embedded images might
want to mark themselves unfreeable, expose a dummy ref_no_free().

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

Access to the gpxe.org and etherboot.org domains and associated
resources has been revoked by the registrant of the domain.  Work
around this problem by renaming project from gPXE to iPXE, and
updating URLs to match.

Also update README, LOG and COPYRIGHTS to remove obsolete information.

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.

in code defining reference-counted objects.