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.
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 'yawning/bug_8402'