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.

unix/plink.c

@@ -394,8 +394,8 @@ static SeatPromptResult plink_get_userpass_input(Seat *seat, prompts_t *p)
 
 static bool plink_seat_interactive(Seat *seat)
 {
-    return (!*conf_get_str(conf, CONF_remote_cmd) &&
-            !*conf_get_str(conf, CONF_remote_cmd2) &&
+    return (!*conf_get_str_ambi(conf, CONF_remote_cmd, NULL) &&
+            !*conf_get_str_ambi(conf, CONF_remote_cmd2, NULL) &&
             !*conf_get_str(conf, CONF_ssh_nc_host));
 }