Allow ClientTransportPlugins to use proxies

This change allows using Socks4Proxy, Socks5Proxy and HTTPSProxy with
ClientTransportPlugins via the TOR_PT_PROXY extension to the
pluggable transport specification.

This fixes bug #8402.
Fixed the test build with --enable-gcc-warnings
Remove get_bridge_pt_addrport().

The code was not disambiguating ClientTransportPlugin configured and
not used, and ClientTransportPlugin configured, but in a failed state.

The right thing to do is to undo moving the get_transport_by_addrport()
call back into get_proxy_addrport(), and remove and explicit check for
using a Bridge since by the time the check is made, if a Bridge is
being used, it is PT/proxy-less.
Log the correct proxy type on failure.

get_proxy_addrport fills in proxy_type with the correct value, so there
is no point in logging something that's a "best guess" based off the
config.
Improve the log message when a transport doesn't support proxies.

Per feedback, explicltly note that the transport will be killed when it
does not acknowledge the configured outgoing proxy.
Divide torrc.sample into torrc.sample and torrc.minimal

torrc.minimal is now the one that should change as infrequently as
possible.  To schedule an change to go into it eventually, make your
change to torrc.minimal.in-sample.

torrc.sample is now the volatile one: we can change it to our hearts'
content.

Closes ticket #11144
Fix: typo in torrc.sample.in
Use 'Bytes', not 'B', in torrc.sample.

Also put this change into torrc.minimal.in-staging

Closes ticket 10343
Fix issues brought up in nickm's review.

 * Update pt_get_proxy_uri() documentation.
 * proxy_supported is now unsigned.
 * Added a changes file.
Merge remote-tracking branch 'public/torrc_minimal'
Merge remote-tracking branch 'yawning/bug_8402'