Cocoa back-end: Replace non-portable blocks with portable lambdas

[barracuda156]

Some Cocoa code uses blocks, which breaks the build with gcc. For example:

mlterm/uitoolkit/quartz/cocoa.m

Lines 121 to 127 in c26445e

	dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
	ui_event_source_process(); /* See ui_event_source-cocoa.m */
	dispatch_sync(dispatch_get_main_queue(), ^{
	exit_program();
	});
	});
	}

These can be replaced with portable lambdas, retaining the functionality, but improving portability.