add a quick-and-dirty method of debugging a single query

add a quick-and-dirty method of debugging a single query

when built with "configure --enable-singletrace", named will produce
detailed query logging at the highest debug level for any query with
query ID zero.

this enables monitoring of the progress of a single query by specifying
the QID using "dig +qid=0". the "client" logging category should be set
to a low severity level to suppress logging of other queries. (the
chance of another query using QID=0 at the same time is only 1 in 2^16.)

"--enable-singletrace" turns on "--enable-querytrace" as well, so if the
logging severity is not lowered, all other queries will be logged
verbosely as well. compiling with either of these options will impair
query performance; they should only be turned on when testing or
troubleshooting.