Functions to access more fields of struct event.

Functions to access more fields of struct event.

Once event_assign() or event_new() had been called, there was no way
to get at a copy of the event's callback, callback argument, or
configured events.  This patch adds an accessor function for each, and
an all-fields accessor for code that wants to re-assign one field of
an event.

This patch also adds a function to return sizeof(struct event), so
that code with intense RAM needs can still retain ABI compatibility
between versions of Libevent without having to heap-allocate every
struct event individually.

The code here was first proposed by Pavel Pisa.