Add nolog option to avoid syslog(3)

Add nolog option to avoid syslog(3)

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.

OK tedu martijn