Initial version with a _very_ raw support for HTTP proxy chains (using the
CONNECT method). This is mostly intended to validate the concepts, data models
and programming approach.
Added a --proxies <chain> option to nmap to let users specify proxies to use
from command line.

<chain> is expected (by nsock) to be a comma-separated list of proxies.
e.g.:
  http://relay1.local:8080,http://relay2.local:3128

This option has currently no effect.
Added proxy support to service_scan.
Design improvements.

Externally:
  The calling application can build a proxychain object and assign it to one (or
  more) NSP. Once a NSP get assigned a proxychain it's not possible to remove
  it so that consistency is (should be...) ensured.

Internally:
  An IOD comes with a proxychain context structure storing the whole tunnel
  state. Also each proxy type now has a table of associated functions to use
  as hooks for TCP connects(), read() and write() requests. As a result, adding
  support of new proxy type should be easier. Code also gains readability in
  comparison to large switch/cases that redirect the execution flow according to
  the given proxy type.
Added destructor for proxy_chain_context.
Reduced scope of the top level proxy event handler.

There is no need anymore for exporting this function to the rest of the library.
It's called by the proxy hooks when registering new events.
Added missing initializer.
Use the new API that nsock provides for proxychain parsing/handling.
Fixed ProxyAction extern declaration.
Removed undesired debugging printf() call.
Cosmetic fixes.
Renamed proxy actions into proxy op, which seems to be rather more meaningful.
Added nsock_proxy.h to the DEPS list.
Refactored proxy code to use a "strategy" design pattern.

Proxy backends are selected at runtime. Each proxy exports a list of operations
to the rest of the library. This is similar to the way IO engines are
implemented within nsock.
Removed the global ProxyOps list.

Each proxy node now provides a pointer to its operations struct.
Removed unused macros.