base: call Io_progress_handler only in signal RPC

This remove the call to Io_progress_handler::handle_io_progress() from
wait_and_dispatch_one_io_signal() to prevent unexpected nesting
I/O-progress handling in case of custom dispatch loops (e.g., in libc).
The original intention of Io_progress_handler was to inform the
entrypoint just before blocking in the entrypoint loop.

Issue #2635

repos/base/src/lib/base/entrypoint.cc

@@ -233,8 +233,6 @@ bool Entrypoint::_wait_and_dispatch_one_io_signal(bool const dont_block)
 		}
 	}
 
-	_handle_io_progress();
-
 	/* initiate potential deferred-signal handling in entrypoint */
 	if (_deferred_signals.first()) {
 		/* construct the handler on demand (otherwise we break core) */

repos/libports/src/lib/libc/internal/kernel.h

@@ -458,6 +458,7 @@ struct Libc::Kernel final : Vfs::Io_response_handler,
 				} else {
 					/* block for signals */
 					_env.ep().wait_and_dispatch_one_io_signal();
+					handle_io_progress();
 				}
 
 				if (!_kernel_routine && _resume_main_once && !_setjmp(_kernel_context))