In theory, we support proxies over HTTPS.  Test that we can support this
_in fact_.

Data-driven tests

sha1dc: update to fix endianess issues on AIX/HP-UX

Add NTLM support for HTTP(s) servers and proxies

Update our copy of sha1dc to the upstream commit 855827c (Detect
endianess on HP-UX, 2019-05-09). Changes include fixes to endian
detection on AIX and HP-UX systems as well as a define that
allows us to force aligned access, which we're not using yet.

When we send HTTP credentials but the server rejects them, tear down the
authentication context so that we can start fresh.  To maintain this
state, additionally move all of the authentication handling into
`on_auth_required`.

When we're issuing a CONNECT to a proxy, we expect to keep-alive to the
proxy.  However, during authentication negotiations, the proxy may close
the connection.  Reconnect if the server closes the connection.

When we have a keep-alive connection to the server, that server may
legally drop the connection for any reason once a successful request and
response has occurred.  It's common for servers to drop the connection
after some amount of time or number of requests have occurred.

We stop the read loop when we have read all the data.  We should also
consider the server's feelings.

If the server hangs up on us, we need to stop our read loop.  Otherwise,
we'll try to read from the server - and fail - ad infinitum.