Allow to limit zed's syslog chattiness

Some usage patterns like send/recv of replication streams can
produce a large number of events. In such a case, the current
all-syslog.sh zedlet will hold up to its name, and flood the
logs with mostly redundant information. Two mitigate this
situation, this changeset introduces to new variables
ZED_SYSLOG_SUBCLASS_INCLUDE and ZED_SYSLOG_SUBCLASS_EXCLUDE
to zed.rc that give more control over which event classes end
up in the syslog.

Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Daniel Kobras <d.kobras@science-computing.de>
Closes #6886
Closes #7260