manuals: System message vs kernel message

Wordsmith mentions throughout the manual of syslog and dmesg, to clearly
differentiate them in an externally consisteny way, increasing operator
onboarding speed and elegance.

The daemon that handles general system messages, syslog, describes them
as "system messages", and "messages" is the standard filename. Rewrite
syslog related manual titles to align search results with this, and hier
entries to align the index. Use care to maintain keywords and not add
extra lines. Newsyslog trades "maintain" with "rotate" for visibility.

MFC after:	3 days
Reviewed by:	markj
Closes:		https://github.com/freebsd/freebsd-src/pull/2067

share/man/man7/hier.7

@@ -327,7 +327,7 @@ Explicitly distrusted certificates; see
 kernel state defaults; see
 .Xr sysctl.conf 5
 .It Pa syslog.conf
-system log configuration
+system message log configuration
 .It Pa ttys
 tty creation configuration; see
 .Xr getty 8
@@ -851,7 +851,7 @@ default log for system daemons
 .It Pa devd.log
 default log for device state change daemon
 .It Pa dmesg.today
-system message buffer log, rotates to
+kernel message buffer log, rotates to
 .Pa dmesg.yesterday
 .It Pa debug.log
 undiscarded debug syslog messages
@@ -862,7 +862,7 @@ logs for the line printer spooler daemon; see
 .Xr sendmail 8
 log, rotates and compresses to maillog.0.bz2
 .It Pa messages
-general system log; see
+general system message log; see
 .Xr syslogd 8
 .It Pa mount.today
 currently loaded