doas(1) unconditionally logs all executions but syslog.conf(5) provides no
means to filter messages by user, target or command.
Add the "nolog" option to doas.conf(5) such that syslog becomes an opt-out
feature; this keeps configuration simple enough yet powerful since rule
definition is the best place to decide whether to log commands or not on a
per rule basis - this also aoids duplicating information or logic in any
other log processing tool.