In general, the "kern" facility is reserved for the kernel use only.
If a program specifies that facility, then it is silently converted
to "user" facility.
So, using logger -p kern.xxx was both misleading and non-specific.
Thus, change the facility to local7, so that users can create
more adequate syslogd configurations.
While local0..local7 are documented as being for local use we already
have several examples in the tree where they are used because none of
the named facilities really fits.
Approved by: asomers
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D12420
Also, extend with some new events.
Additionally, tag syslog messages with 'ZFS' instead of using explicit "ZFS: "
prefix in the messages themselves.
Tested by: Daniel O'Connor <doconnor@gsoft.com.au>
MFC after: 8 days
Sponsored by: HybridCluster
