Replace the libwinpty API.

The new API improves upon the old API in a number of ways:

 * The old API provided a single data pipe for input and output, and it
   provided the pipe in the form of a HANDLE opened with
   FILE_FLAG_OVERLAPPED.  Using a bare HANDLE is difficult in higher-level
   languages, and overlapped/asynchronous I/O is hard to get right.
   winpty_close closed the data pipe, which also didn't help things, though
   I think the handle could have been duplicated.

   Using a single handle for input and output complicates shutdown.  When
   the child process exits, the agent scrapes the final output, then closes
   the data pipe once its written.  It's possible that a winpty client will
   first detect the closed pipe when it's writing *input* rather than
   reading output, which seems wrong.  (On the other hand, the agent
   doesn't implement "backpressure" for either input or output (yet), and
   if it did, it's possible that post-exit shutdown should interrupt a
   blocked write into the console input queue.  I need to think about it
   more.)

   The new API splits the data pipe into CONIN and CONOUT pipes, which are
   accessed by filename.  After `winpty_open` returns, the client queries
   pipe names using `winpty_conin_name` and `winpty_conout_name`, then
   opens them using any mechanism, low-level or high-level, blocking or
   overlapped.

 * The old libwinpty handled errors by killing the process.  The new
   libwinpty uses C++ exceptions internally and translates exceptions at
   API boundaries using:

    - a boolean error result (e.g. BOOL, NULL-vs-non-NULL)
    - a potentially heap-allocated winpty_error_t object returned via an
      optional winpty_error_ptr_t parameter.  That parameter can be NULL.
      If it isn't, then an error code and message can be queried from the
      error object.  The winpty_error_t object must be freed with
      winpty_error_free.

 * winpty_start_process is renamed to winpty_spawn.  winpty_open and
   winpty_spawn accept a "config" object which holds parameters.  New
   configuration options can be added without disturbing the source or
   binary API.

 * The winpty_get_exit_code and winpty_get_process_id APIs are removed.
   The winpty_spawn function has an out parameter providing the child's
   process and thread HANDLEs (duplicated from the agent via
   DuplicateHandle).  The winpty client can call GetExitCodeProcess and
   GetProcessId (as well as the WaitForXXX APIs to wait for child exit).

Makefile

@@ -131,6 +131,7 @@ install-doc :
 install-include :
 	mkdir -p $(PREFIX)/include/winpty
 	install -m 644 -p src/include/winpty.h $(PREFIX)/include/winpty
+	install -m 644 -p src/include/winpty_constants.h $(PREFIX)/include/winpty
 
 .PHONY : install
 install : \

RELEASES.md

@@ -1,3 +1,7 @@
+# Next Version
+
+The winpty library has a new API that should be easier for embedding.
+
 # Version 0.3.0 (2016-05-20)
 
 User-visible changes: