If gPXE FILE_READ returns an actual error, don't loop forever.

Add gPXE PXENV_FILE_API_CHECK call, and rationalize the pxenv wrapper
subroutine.  We now always accept the return value in AX and convert
it to a carry, and we always preserve all registers.

Don't rely on PXENV_GET_FILE_SIZE in gPXE support mode.  It sometimes
successfully returns wrong information.

With this patch, and the very latest gPXE tree from today, I can
download my usual boot setup via http.

Remove debugging code which prevents building with tracers on.

Clean up merge issues between the nolen and gpxe-support branches.