/etc/rc.subr: add support for kld sysctl variables

For kernel modules loaded by scripts in /etc/rc.d and
/usr/local/etc/rc.d, if there is a file in /etc/sysctl.conf.d named <kld
name>.conf, then this will be loaded using the sysctl(8) utility. For
instance, sysctl variable changes for the pf kernel module would be
placed in the file /etc/sysctl.conf.d/pf.conf.

PR:		272129
Reviewed by:	imp freebsd_igalic.co
MFC after:	2 weeks
Differential Revision: https://reviews.freebsd.org/D40886

etc/mtree/BSD.root.dist

@@ -96,6 +96,8 @@
         ..
         ssl
         ..
+	sysctl.conf.d
+	..
         syslog.d
         ..
         zfs

libexec/rc/rc.subr

@@ -1970,6 +1970,9 @@ load_kld()
 			return 1
 		else
 			info "$1 kernel module loaded."
+			if [ -f "/etc/sysctl.conf.d/$1.conf" ]; then
+				sysctl -f "/etc/sysctl.conf.d/$1.conf"
+			fi
 		fi
 	else
 		debug "load_kld: $1 kernel module already loaded."

share/man/man5/sysctl.conf.5

@@ -51,6 +51,13 @@ Comments can also exist at the end of a line,
 as seen in the
 .Sx EXAMPLES
 section, below.
+.Pp
+For kernel modules loaded via
+.Xr rc.subr 8
+system ,
+additional module-specific settings can be applied
+by adding a file in the same format named
+.Pf /etc/sysctl.d/<module name>.conf .
 .Sh FILES
 .Bl -tag -width /etc/rc.d/sysctl_lastload -compact
 .It Pa /etc/rc.d/sysctl
@@ -69,6 +76,9 @@ Initial settings for
 .It Pa /etc/sysctl.conf.local
 Machine-specific settings for sites with a common
 .Pa /etc/sysctl.conf .
+.It Pa /etc/sysctl.conf.d
+Module specific settings for kernel modules loaded via
+.Xr rc.subr 8 .
 .El
 .Sh EXAMPLES
 To turn off logging of programs that exit due to fatal signals you may use