Switch CONF_remote_cmd to being STR_AMBI.

The immediate usefulness of this is in pterm.exe: when the user uses
-e to specify a command to run in the pterm, we retrieve the command
in Unicode, store it in CONF_remote_cmd as UTF-8, and then in conpty.c
we can extract it in the same form and convert it back to Unicode to
pass losslessly to CreateProcessW. So now non-ACP Unicode works in
that part of the pterm command line.

test/test_conf.c

@@ -696,7 +696,7 @@ void test_simple(void)
     test_str_simple(CONF_proxy_telnet_command, "ProxyTelnetCommand", "connect %host %port\\n");
     test_int_translated(CONF_proxy_log_to_term, "ProxyLogToTerm", FORCE_OFF,
                         FORCE_ON, 0, FORCE_OFF, 1, AUTO, 2, -1);
-    test_str_simple(CONF_remote_cmd, "RemoteCommand", "");
+    test_str_ambi_simple(CONF_remote_cmd, "RemoteCommand", "", false);
     test_bool_simple(CONF_nopty, "NoPTY", false);
     test_bool_simple(CONF_compression, "Compression", false);
     test_bool_simple(CONF_ssh_prefer_known_hostkeys, "PreferKnownHostKeys", true);