Add --with-group= option to config.ac

The powerman.service file has configuration options to run the daemon
as a specific user and group. The current default for both is "daemon".
Changing the user is possible through --with-user= configure option,
but there is no option for changing the group.

In order to allow the group value to be configured at build time, a
configure option similar to the --with-user= option should be provided.

This change creates the --with-group= option, setting the
@RUN_AS_GROUP@ macro for autoconf files.

Note from @garlick: this was submitted in 2017 as pr chaos#26 and sat around
due to maintainer inattention.  Rebasing and reviving as it's a good patch!

config/ac_runas.m4

@@ -1,6 +1,7 @@
 AC_DEFUN([AC_RUNAS],
 [
     RUN_AS_USER="daemon"
+    RUN_AS_GROUP="daemon"
     AC_MSG_CHECKING(user to run as)
     AC_ARG_WITH(user,
     AS_HELP_STRING([--with-user=username], [user for powerman daemon (daemon)]),
@@ -16,4 +17,20 @@ AC_DEFUN([AC_RUNAS],
             [Powerman daemon user])
     AC_MSG_RESULT(${RUN_AS_USER})
     AC_SUBST(RUN_AS_USER)
+
+    AC_MSG_CHECKING(group to run as)
+    AC_ARG_WITH(group,
+    AC_HELP_STRING([--with-group=groupname], [group for powerman daemon (daemon)]),
+    [       case "${withval}" in
+            yes|no)
+                    ;;
+            *)
+                    RUN_AS_GROUP="${withval}"
+                    ;;
+            esac],
+    )
+    AC_DEFINE_UNQUOTED(RUN_AS_GROUP, "${RUN_AS_GROUP}",
+            [Powerman daemon group])
+    AC_MSG_RESULT(${RUN_AS_GROUP})
+    AC_SUBST(RUN_AS_GROUP)
 ])

configure.ac

@@ -109,7 +109,7 @@ AC_DEFINE(WITH_LSD_NOMEM_ERROR_FUNC, 1, [Define lsd_fatal_error])
 # whether to install pkg-config file for API
 AC_PKGCONFIG
 
-# what user to run daemon as
+# what user and group to run daemon as
 AC_RUNAS
 
 ##

etc/powerman.service.in

@@ -6,8 +6,8 @@ After=syslog.target network.target
 Environment=SHELL=/bin/sh
 Type=forking
 PrivateTmp=yes
-User=daemon
-Group=daemon
+User=@RUN_AS_USER@
+Group=@RUN_AS_GROUP@
 ExecStart=@X_SBINDIR@/powermand
 PIDFile=@X_RUNSTATEDIR@/powerman/powermand.pid