Fix handling of passwords in URLs

So far, passwords in URLs (eg http://user:password@host..) have been
handled as part of the username, and thus passing
  add-drive path username:username:password ...
instead of
  add-drive path username:username secret:password ...

Fix the parsing of URLs to handle passwords as separate elements,
properly passing it as "secret" parameter for add-drive, and properly
readd it when building URLs in the direct backend.

Furthmore, to keep curl- and ssh-based qemu drivers working with
authenticated resources, make sure they can accept secrets.

Reported in comment #1 of RHBZ#1092583.
test-charset-fidelity: allow to skip testing specific FSes

Allow to skip testing the filesystem "foo" if the environment variable
SKIP_TEST_CHARSET_FIDELITY_foo=1 is set. This way it possible to not
test one or more filesystems without disabling the test altogether.