Patch #1520294: Support for getset and member descriptors in types.py,

inspect.py, and pydoc.py.  Specifically, this allows for querying the type of
an object against these built-in C types and more importantly, for getting
their docstrings printed in the interactive interpreter's help() function.

This patch includes a new built-in module called _types which provides
definitions of getset and member descriptors for use by the types.py module.
These types are exposed as types.GetSetDescriptorType and
types.MemberDescriptorType.  Query functions are provided as
inspect.isgetsetdescriptor() and inspect.ismemberdescriptor().  The
implementations of these are robust enough to work with Python implementations
other than CPython, which may not have these fundamental types.

The patch also includes documentation and test suite updates.

I commit these changes now under these guiding principles:

1. Silence is assent.  The release manager has not said "no", and of the few
   people that cared enough to respond to the thread, the worst vote was "0".

2. It's easier to ask for forgiveness than permission.

3. It's so dang easy to revert stuff in svn, that you could view this as a
   forcing function. :)

Windows build patches will follow.

Doc/lib/libinspect.tex

@@ -180,13 +180,32 @@ Note:
   Return true if the object is a data descriptor.
 
   Data descriptors have both a __get__ and a __set__ attribute.  Examples are
-  properties (defined in Python) and getsets and members (defined in C).
-  Typically, data descriptors will also have __name__ and __doc__ attributes 
-  (properties, getsets, and members have both of these attributes), but this 
-  is not guaranteed.
+  properties (defined in Python), getsets, and members.  The latter two are
+  defined in C and there are more specific tests available for those types,
+  which is robust across Python implementations.  Typically, data descriptors
+  will also have __name__ and __doc__ attributes (properties, getsets, and
+  members have both of these attributes), but this is not guaranteed.
 \versionadded{2.3}
 \end{funcdesc}
 
+\begin{funcdesc}{isgetsetdescriptor}{object}
+  Return true if the object is a getset descriptor.
+
+  getsets are attributes defined in extension modules via \code{PyGetSetDef}
+  structures.  For Python implementations without such types, this method will
+  always return \code{False}.
+\versionadded{2.5}
+\end{funcdesc}
+
+\begin{funcdesc}{ismemberdescriptor}{object}
+  Return true if the object is a member descriptor.
+
+  Member descriptors are attributes defined in extension modules via
+  \code{PyMemberDef} structures.  For Python implementations without such
+  types, this method will always return \code{False}.
+\versionadded{2.5}
+\end{funcdesc}
+
 \subsection{Retrieving source code
             \label{inspect-source}}