rc: dmesg: Allow umask to be configurable

Allow umask to be configurable.

Being able to set the umask via an rc variable is useful when setting:

    security.bsd.unprivileged_read_msgbuf=0

As it allows a user to configure:

    dmesg_umask="066"

Without modifying the rc script, and preventing the contents of the
$dmesg_file (/var/run/dmesg.boot) from being publicly readable.

PR:		272552
Reviewed by:	netchild
MFC after:	2 days
Differential Revision:	https://reviews.freebsd.org/D53169

libexec/rc/rc.conf

@@ -707,6 +707,7 @@ osrelease_enable="YES"	# Update /var/run/os-release on boot (or NO).
 osrelease_file="/var/run/os-release" # File to update for os-release.
 osrelease_perms="444"	# Default permission for os-release file.
 dmesg_enable="YES"	# Save dmesg(8) to /var/run/dmesg.boot
+dmesg_umask="022"	# Default umask for /var/run/dmesg.boot file.
 watchdogd_enable="NO"	# Start the software watchdog daemon
 watchdogd_flags=""	# Flags to watchdogd (if enabled)
 watchdogd_timeout=""	# watchdogd timeout, overrides -t in watchdogd_flags

libexec/rc/rc.d/dmesg

@@ -19,7 +19,7 @@ stop_cmd=":"
 do_dmesg()
 {
 	rm -f ${dmesg_file}
-	( umask 022 ; /sbin/dmesg $rc_flags > ${dmesg_file} )
+	( umask "${dmesg_umask}" ; /sbin/dmesg $rc_flags > ${dmesg_file} )
 }
 
 load_rc_config $name