units: do not ignore return value from systemd --user

This minus has been there since the unit was added in
d42d27ead91e470cb12986d928441e56c0f543ca. I think the idea was not cause things
to fail if the user instance doesn't work. But ignoring the return value
doesn't seem to be the right way to approach the problem. In particular, if
the program fails to run, we'll get a bogus fail state, see
https://bugzilla.redhat.com/show_bug.cgi?id=1727895#c1:

with the minus:
$ systemctl start user@1002
Job for user@1002.service failed because the service did not take the steps required by its unit configuration.
See "systemctl status user@1002.service" and "journalctl -xe" for details.

without the minus:
$ systemctl start user@1002
Job for user@1002.service failed because the control process exited with error code.
See "systemctl status user@1002.service" and "journalctl -xe" for details.

src/core/main.c

@@ -1930,7 +1930,7 @@ static int initialize_runtime(
                         status_welcome();
                         hostname_setup();
                         machine_id_setup(NULL, arg_machine_id, NULL);
-                        loopback_setup();
+                        (void) loopback_setup();
                         bump_unix_max_dgram_qlen();
                         bump_file_max_and_nr_open();
                         test_usr();

src/nspawn/nspawn.c

@@ -3000,7 +3000,7 @@ static int inner_child(
                 return log_error_errno(errno, "setsid() failed: %m");
 
         if (arg_private_network)
-                loopback_setup();
+                (void) loopback_setup();
 
         if (arg_expose_ports) {
                 r = expose_port_send_rtnl(rtnl_socket);

units/user@.service.in

@@ -18,7 +18,7 @@ IgnoreOnIsolate=yes
 User=%i
 PAMName=systemd-user
 Type=notify
-ExecStart=-@rootlibexecdir@/systemd --user
+ExecStart=@rootlibexecdir@/systemd --user
 Slice=user-%i.slice
 KillMode=mixed
 Delegate=pids memory