PXELINUX: handle OACK packets with extra NULs, cleaner TFTP error

PXELINUX: handle OACK packets with extra NULs, cleaner TFTP error

There are apparently TFTP servers in the field which will send OACK
packets with extra NUL bytes appended at the end.  If we find an OACK
packet where the only thing left at some point during processing is
NULs, then just consider the packet processed.

We have reported all TFTP protocol errors as "tsize required", which
is definitely not true anymore.  Change error code to 0 (undefined)
and the error string to "TFTP error".  When this code gets converted
to C we'll do better.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>