opencl: correctly test if no kernel name was specified in config.

Closes second bug squashed into https://github.com/veox/sgminer/issues/123
report.
log: mention non-user-settable kernel parameters in "Initialising kernel" notice.

Should help in the future for people who try to mix patched and unpatched
BFI devices.
log: warn on using kernel zuikkis with lookup-gap != 2.
opencl: reuse strbuf in initCl, get rid of poorly-named numbuf.
api: remove unused SCRYPTSTR.

Gets rid of compiler warning.