util/log: Support per-thread log files

Add a new log flag, tid, to turn this feature on.
Require the log filename to be set, and to contain %d.

Do not allow tid to be turned off once it is on, nor let
the filename be change thereafter.  This avoids the need
for signalling each thread to re-open on a name change.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220417183019.755276-40-richard.henderson@linaro.org>