sysctl.d: Include a README file for /etc/sysctl.d/

Our sysctl script has read from /etc/sysctl.d/ but the directory was not
created by default, and we didn't document it.

X-Gentoo-Bug: 398189
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=398189
Signed-off-by: Robin H. Johnson <robbat2@gentoo.org>

Makefile

@@ -4,7 +4,7 @@
 
 include Makefile.inc
 
-SUBDIR=		conf.d doc etc init.d local.d man net scripts sh src
+SUBDIR=		conf.d doc etc init.d local.d sysctl.d man net scripts sh src
 
 # Build pkgconfig or not
 MKPKGCONFIG?=	yes

mk/sys.mk

@@ -22,6 +22,7 @@ SYSCONFDIR?=		${PREFIX}/etc
 INITDIR?=		${SYSCONFDIR}/init.d
 CONFDIR?=		${SYSCONFDIR}/conf.d
 LOCALDIR?=		${SYSCONFDIR}/local.d
+SYSCTLDIR?=		${SYSCONFDIR}/sysctl.d
 
 BINDIR?=		${PREFIX}/sbin
 BINMODE?=		0755

sysctl.d/Makefile

@@ -0,0 +1,6 @@
+DIR=	${SYSCTLDIR}
+CONF=	README
+
+MK=		../mk
+include ${MK}/os.mk
+include ${MK}/scripts.mk

sysctl.d/README

@@ -0,0 +1,13 @@
+Kernel system variables configuration files
+
+Files found under the /etc/sysctl.d directory that end with .conf are
+parsed within sysctl(8) at boot time.  If you want to set kernel variables
+you can either edit /etc/sysctl.conf or make a new file.
+
+The filename isn't important, but don't make it a package name as it may clash
+with something the package builder needs later. It must end with .conf or else
+it will not be read.
+
+The recommended locationfor local system settings is /etc/sysctl.d/local.conf
+but as long as you follow the rules for the names of the file, anything will
+work. See sysctl.conf(5) man page for details of the format.