debian: install udev rule and create related group

postinst script based on the example here:
https://www.debian.org/doc/manuals/securing-debian-manual/bpp-lower-privs.en.html

Signed-off-by: Oliver Smith <osmith@sysmocom.de>

debian/control

@@ -12,7 +12,7 @@ Homepage: http://sispmctl.sourceforge.net/
 
 Package: sispmctl
 Architecture: any
-Depends: ${shlibs:Depends}, ${misc:Depends}
+Depends: ${shlibs:Depends}, ${misc:Depends}, adduser (>= 3.11)
 Description: Control Gembird SIS-PM programmable power outlet strips
  The sispmctl tool can control Gembird SIS-PM Silver Shield programmable
  power outlet strips (also known under the name Revolt Intelli-Plug) from

debian/sispmctl.install

@@ -1,2 +1,3 @@
 /usr/bin/sispmctl
 /usr/share/man/man1/sispmctl.1
+/usr/share/doc/sispmctl/examples/60-sispmctl.rules /lib/udev/rules.d/

debian/sispmctl.postinst

@@ -0,0 +1,11 @@
+#!/bin/sh
+
+case "$1" in
+  configure)
+  if ! getent group | grep -q "^sispmctl:" ; then
+     echo -n "Adding group sispmctl.."
+     addgroup --quiet --system sispmctl 2>/dev/null ||true
+     echo "..done"
+  fi
+  ;;
+esac