FS, ISO9600: mtime; HTTP: send ETag.

kernel/fs.s:
	add fs_handle_get_mtime.

kernel/fs/iso9660.s:
	parse RockRidge extensions on file open aswell,
	and copy the mtime to fs dirent's posix_mtime.

kernel/net/httpd.s:
	allox 8 extra bytes for file buffer and use them to store the mtime,
	and send the mtime as ETag header (since that is easier than
	calculating the day-of-week for a date which is required for a Date
	header).
HTTPD: support If-None-Match caching

kernel/lib/string.s:
	make htoid global

kernel/net/httpd.s:
	allocate stack variables for headers;

	detect If-None-Match header, and if the value matches the file mtime,
	send 304 Not Modified without allocating file buffer or reading file
	contents.

	(responses are served in 3..5 ms - the PIT schedule resolution!).