Original commit by Yawning Angel.

Per discussion in tor bug #8402.

A from the spec implementation of TOR_PT_PROXY and the related APIs.

Added:
 * ClientTransportPlugin.reportProxySuccess(self) - Report proxy
   config success.
 * ClientTransportPlugin.reportProxyError(self, msg) - Report proxy
   config error.
 * ClientConfig.getProxy(self) - Obtain the configured proxy as a
   urlparse.urlsplit tuple (urlparse.SplitResult).  This will return
   None if no proxy is configured.

After calling urlsplit the code will correctly validate that it is in a
form specified by checking:
 * scheme is one of http, socks4a, socks5.
 * hostname is a valid IP address (No resolution is done).
 * port is a valid port.
 * No path, query, or fragment is present.
 * Only the username is specified for socks4a.

See also:
 * https://trac.torproject.org/projects/tor/ticket/8402
 * https://gitweb.torproject.org/torspec.git/blob/HEAD:/proposals/ideas/xxx-pluggable-transports-through-proxy.txt

- potential strategy to fix similar to https://trac.torproject.org/projects/tor/ticket/9330#comment:11 but using CTRL-C for SIGINT
- also probably requires python fix for http://bugs.python.org/issue18040

- potential strategy to fix at https://trac.torproject.org/projects/tor/ticket/9330#comment:11

It looks like this code might have been originally written to use a
dict, and the mswindows branch didn't get updated to work with a list of
tuples instead. This is the exception I was getting on Windows:

Traceback (most recent call last):
  File "obfs-flash-client", line 11, in <module>
    from pyptlib.util.subproc import auto_killall, Popen
  File "pyptlib\util\subproc.py", line 31, in <module>
    _Popen_defaults['creationflags'] |= subprocess.CREATE_NEW_PROCESS_GROUP
TypeError: list indices must be integers, not str

They actually parse and return values; they don't just validate.

- idea imported from previous API but is unnecessary and just adds complexity
- instead, only expose the latter in the API

declareSupports - make managedTransportVer like a tweakable setting that may be used in future, and remove the unnecessary return value

- deprecate old API that created a new config for each method call

- add a unified Config.declareSupports() method for use in initialisation
  - remove most of the check* methods, instead do this directly in declareSupports()
  - move common logic from {client,server}.init() into declareSupports()
- add a unified envvar validation system using composable first-class functions
  - exceptions thrown during validation automatic trigger config.writeEnvError()