www-cgi/: Add some trivial tracing.

www-cgi/: Add some trivial tracing.

The trace goes to standard output, and only happens when debugging is
turned on, both operationally (e.g., though the `ucgi-debug' link or
USERV_U_DEBUG variable) and at compile time (with the DEBUG macro, which
you can set, e.g., with `make DEBUG="-g -DDEBUG"'.

Trace output lines start with `;;'.

I'll be grateful for this when I start shaking things up.