Rename handler to module, callback to handler

This is hopefully less confusing.

Quick idea: instead of having
	prefix $foo {
		module = $bar
		arg1 = val1
		...
		argn = valn
	}

We could have as well:
	$bar $foo {
		arg1 = val1
		...
		argn = valn
	}

This will be easier as soon as we have a module registry or something
like that.
Add simple module registry
Move redirect code from lwan.c to a redirect module