nassh: expose connected hostname as a URL query parameter

Chrome Enterprise data controls (e.g. clipboard DLP rules) are keyed
off the page URL, so record the ssh host we connect to as
?canonicalized-hostname=<host>.  The parameter is output only; nassh
never reads it.  The user-facing contract is documented in the FAQ
alongside the rest of the nassh.html URI spec.

navigate_() is unchanged: it still swaps the #hash via
location.replace(), which is a same-document (fragment) navigation.
The query string can't take that path -- a query change through
location.replace() is a cross-document navigation that would reload
the page and kill the session -- so a new setHostnameParam_() helper
updates it with history.replaceState() instead: no reload and no new
history entry.

The hostname is written twice.  First from the profile/destination
before connecting, i.e. whatever the user typed.  Then, since
ssh_config may rewrite that (HostName) or it may be an IP literal, a
new onConnectAttempt hook in the wassh syscall handler refines it with
the address ssh actually tries to reach on its first AF_INET/AF_INET6
connect.  The hook only touches the URL in place; it never navigates,
and exceptions in it cannot affect the connect.

Change-Id: I131f0bf0907f4309b5a808b13986ff9a69660001
Reviewed-on: https://chromium-review.googlesource.com/c/apps/libapps/+/8487515
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: kokoro <noreply+kokoro@google.com>