introduce support for sending the If-Modified-Since header while

introduce support for sending the If-Modified-Since header while
fetching over http(s) and use the timestamps from the remote server's
Last-Modified header if available when saving local files
this makes it possible to mirror files better with ftp(1)

the new timestamp behaviour can be disabled with the new '-u' flag

ok sthen@, input from sthen@ and gnezdo@