shared/log: add log_trace as compile-time optional debugging

Repetetive messages can be annoying when running with
SYSTEMD_LOG_LEVEL=debug, but they are sometimes very useful
when debugging problems. Add log_trace which is like log_debug
but becomes a noop unless LOG_TRACE is defined during compilation.
This makes it easy to enable very verbose logging for a subset
of programs when compiling from source.

Signed-off-by: Anthony G. Basile <blueness@gentoo.org>