shell: Don't encode breadcrumb in URL.

Instead, show a global breadcrumb that always looks like

  Hosts >> $hostname >> $page

The hierarchical navigation functions "go_up", "go_down", and
"go_sibling" have been replaced with "go_rel".

Every page now has a "machine" option that gives the address of the
machine to connect to.

This is an intermediate step, and the next commit restores full
navigation by putting local breadcrumbs into the individual pages that
need them.
shell: Local breadcrumbs.