qrexec: forbid '$' in target and service name

Those parameters eventually may eventually be passed to a shell script
(at least /usr/lib/qubes/qubes-rpc-multiplexer). While it is possible to
properly escape shell special characters, lets do safer and less fragile
thing: forbid such characters entirely.
In case of target name, qrexec policy keywords are allowed, and after
recent change, those contains '@', so allow this char.

(cherry picked from commit cf28dad1943bd424b2ed23000cdcfd32d8e3190c)

qrexec/qrexec-daemon.c

@@ -632,8 +632,7 @@ static void sanitize_name(char * untrusted_s_signed, char *extra_allowed_chars)
             continue;
         if (*untrusted_s >= '0' && *untrusted_s <= '9')
             continue;
-        if (*untrusted_s == '$' ||
-               *untrusted_s == '_' ||
+        if (*untrusted_s == '_' ||
                *untrusted_s == '-' ||
                *untrusted_s == '.')
             continue;
@@ -665,7 +664,7 @@ static void handle_execute_service(void)
     ENSURE_NULL_TERMINATED(untrusted_params.target_domain);
     ENSURE_NULL_TERMINATED(untrusted_params.request_id.ident);
     sanitize_name(untrusted_params.service_name, "+");
-    sanitize_name(untrusted_params.target_domain, "");
+    sanitize_name(untrusted_params.target_domain, "@");
     sanitize_name(untrusted_params.request_id.ident, " ");
     params = untrusted_params;
     /* sanitize end */