Add support for setting effective group ID of services

Finit has long since supported an @user:group syntax to services, tasks,
and run stanzas.  Setting UID was implemented at that time, but setting
GID never was.  This patch fixes that oversight.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

src/service.c

@@ -184,12 +184,14 @@ static int service_start(svc_t *svc)
 		char *home = NULL;
 #ifdef ENABLE_STATIC
 		int uid = 0; /* XXX: Fix better warning that dropprivs is disabled. */
+		int gid = 0;
 #else
 		int uid = getuser(svc->username, &home);
+		int gid = getgroup(svc->group);
 #endif
 		char *args[MAX_NUM_SVC_ARGS];
 
-		/* Set desired user */
+		/* Set desired user+group */
 		if (uid >= 0) {
 			setuid(uid);
 
@@ -202,6 +204,9 @@ static int service_start(svc_t *svc)
 			}
 		}
 
+		if (gid >= 0)
+			setgid(gid);
+
 		/* Serve copy of args to process in case it modifies them. */
 		for (i = 0; i < (MAX_NUM_SVC_ARGS - 1) && svc->args[i][0] != 0; i++)
 			args[i] = svc->args[i];