Add a "ctrl" mechanism to bufferevents for property access.

Add a "ctrl" mechanism to bufferevents for property access.

OpenSSL uses something like this to implement get/set access for
properties on its BIOs, so that it doesn't need to add a pair of
get/set functions to the vtable struct for every new abstract property
it provides an accessor for.

Doing this lets us make bufferevent_setfd abstract, and implement an
abstract bufferevent_getfd.

svn:r1284