previously it was possible to fetchurl a dict server, or an ldap server,
or an imap server. this is a bit of a problem, both because rare schemes
may not be available on all systems, and because some schemes (e.g. scp)
are inherently insecure in potentially surprising ways we needn't allow.
Change-Id: I18fc567c6f58c3221b5ea8ce927f4da780057828
