When generating a Kyuafile in the KYUAFILE=auto case, use a filename
that is unlikely to clash with the filename used by explicitly-provided
Kyuafiles.
This allows a Makefile to set KYUAFILE=yes and provide a Kyuafile in
the same directory when such Makefile was previously relying on
KYUAFILE=auto.
Fixes issues with new Kyuafiles not being picked up in NO_CLEAN builds
(although manual intervention is required once, unfortunately, as
described in UPDATING).
Reviewed by: sjg
MFC after: 1 week
d7efee230d