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.
Mark StrictE{ntry,xit}Nodes as obsolete.
Merge remote-tracking branch 'public/bug12908_025' into maint-0.2.5
Merge remote-tracking branch 'public/torrc_minimal'
Merge remote-tracking branch 'yawning/bug_8402'
In routerlist_assert_ok(), check r2 before taking &(r2->cache_info)

Technically, we're not allowed to take the address of a member can't
exist relative to the null pointer.  That makes me wonder how any sane
compliant system implements the offsetof macro, but let's let sleeping
balrogs lie.

Fixes 13096; patch on 0.1.1.9-alpha; patch from "teor", who was using
clang -fsanitize=undefined-trap -fsanitize-undefined-trap-on-error -ftrapv
Merge remote-tracking branch 'origin/maint-0.2.5'
Avoid overflows and underflows in sscanf and friends

(Patch from teor on 13104)
Extra tests for tor_memeq and memcmp

(Patch from teor; part of 13104)
Avoid divide by zero and NaNs in scale_array_elements_to_u64

Patch from teor; part of 13104
Avoid an overflow on negation in format_helper_exit_status

Part of 13104; patch from teor.
Add a changes file for bug 13104