qrexec: automatically translate '$' to '@' in target name specification

After the change of target keywords, apply transparen translation from
old names. This will avoid breaking calls from not updated VMs. Apply
this change only in stable branch.

qrexec/qrexec-daemon.c

@@ -632,6 +632,10 @@ static void sanitize_name(char * untrusted_s_signed, char *extra_allowed_chars)
             continue;
         if (*untrusted_s >= '0' && *untrusted_s <= '9')
             continue;
+        if (*untrusted_s == '$' && strchr(extra_allowed_chars, '@')) {
+            *untrusted_s = '@';
+            continue;
+        }
         if (*untrusted_s == '_' ||
                *untrusted_s == '-' ||
                *untrusted_s == '.')