weston-launch: always drop privileges before execve

The current code works if pw->pw_shell is bash because:
 "If the shell is started with the effective user (group) id not equal to
 the real user (group) id, and the -p option is not supplied, these actions
 are taken and the effective user id is set to the real user id."

Thus, for bash, weston's EUID == UID.

For zsh, the -p option "is enabled automatically on startup if the effective
user (group) ID is  not equal to the real user (group) ID."
Thus, weston's EUID = 0, and if pw_shell is zsh, /run/user/$UID/wayland-0 is
created with euid root and not writeable by the user, causing all clients to
fail.

Fix this by always dropping privileges to the user.

Regression introduced in 636156d.

src/weston-launch.c

@@ -513,13 +513,6 @@ setup_session(struct weston_launch *wl)
 			error(1, errno, "TIOCSCTTY failed - tty is in use");
 	}
 
-	if (setgid(wl->pw->pw_gid) < 0 ||
-#ifdef HAVE_INITGROUPS
-	    initgroups(wl->pw->pw_name, wl->pw->pw_gid) < 0 ||
-#endif
-	    setuid(wl->pw->pw_uid) < 0)
-		error(1, errno, "dropping privileges failed");
-
 	term = getenv("TERM");
 	clearenv();
 	setenv("TERM", term, 1);
@@ -538,6 +531,17 @@ setup_session(struct weston_launch *wl)
 	}
 }
 
+static void
+drop_privileges(struct weston_launch *wl)
+{
+	if (setgid(wl->pw->pw_gid) < 0 ||
+#ifdef HAVE_INITGROUPS
+	    initgroups(wl->pw->pw_name, wl->pw->pw_gid) < 0 ||
+#endif
+	    setuid(wl->pw->pw_uid) < 0)
+		error(1, errno, "dropping privileges failed");
+}
+
 static void
 launch_compositor(struct weston_launch *wl, int argc, char *argv[])
 {
@@ -550,6 +554,8 @@ launch_compositor(struct weston_launch *wl, int argc, char *argv[])
 	if (wl->new_user)
 		setup_session(wl);
 
+	drop_privileges(wl);
+
 	if (wl->tty != STDIN_FILENO)
 		setenv_fd("WESTON_TTY_FD", wl->tty);