The autoupdater supports HTTPS when a ustream TLS backend is installed,
but we did not allow this in site.conf. However, just allowing HTTPS
URLs unconditionally is also a bad idea, as it might result in nodes
being unable to reach the mirror, in particular if the `tls` feature is
enabled only for some devices.

Solve this by allowing https:// URLs only if the marker file installed
by gluon-tls is found, failing the site check with an error message like
the following otherwise:

    *** All of the following alternatives have failed:
        1) site.conf error: expected autoupdater.branches.test.mirrors.1 to match pattern 'http://', but it is "https://..." (a string value)
        2) site.conf error: expected autoupdater.branches.test.mirrors.1 to use HTTPS only if the 'tls' feature is enabled, but it is "https://..." (a string value)
        3) site.conf error: expected autoupdater.branches.test.mirrors.1 to match pattern '^//', but it is "https://..." (a string value)

In addition, introduce support for protocol-less //server/path URLs,
which will use either HTTP or HTTPS depending on the availablility of
the `tls` feature. No fallback happens when `tls` is available, but the
HTTPS connection fails, preventing downgrade attack.

Based-on-patch-by: Kevin Olbrich <ko@sv01.de>