Fix crash on untrusted XMLRPC connections

Root cause: network.rpc.use_xmlrpc and network.rpc.use_jsonrpc were not
marked as untrusted-safe, but RpcManager::process() calls them before
dispatching to the protocol handler. When an untrusted request arrived,
call_command() threw untrusted_error for these gatekeepers, which escaped
the callback_interrupt_pollling callback and crashed rtorrent.

Fix: Mark network.rpc.use_xmlrpc/jsonrpc as safe (CMD2_VAR_BOOL_U).

Also harden exception safety:
- SCGI callback catch-all now sends a generic error response instead of
  re-throwing, since the callback infrastructure may not support
  exception propagation.
- xmlrpc_c.cc now has catch(std::exception&) and catch(...) safety nets
  after the specific exception handlers.

src/rpc/xmlrpc_c.cc

@@ -396,6 +396,14 @@ xmlrpc_call_command(xmlrpc_env* env, xmlrpc_value* args, void* voidServerInfo) {
   } catch (torrent::local_error& e) {
     xmlrpc_env_set_fault(env, XMLRPC_PARSE_ERROR, e.what());
     return NULL;
+
+  } catch (std::exception& e) {
+    xmlrpc_env_set_fault(env, XMLRPC_PARSE_ERROR, e.what());
+    return NULL;
+
+  } catch (...) {
+    xmlrpc_env_set_fault(env, XMLRPC_PARSE_ERROR, "Unknown exception in command execution.");
+    return NULL;
   }
 }