Backport: provide EVENT_LOG_* names, and deprecate _EVENT_LOG_*

Backport: provide EVENT_LOG_* names, and deprecate _EVENT_LOG_*

This is a partial backport of cb9da0bf and a backport of c9635349.

Because C doesn't like us to declare identifiers starting with an
underscore, Libevent 2.1 has renamed every such identifier.  The
only change that affects a public API is that the _EVENT_LOG_*
macros have been renamed to start with EVENT_LOG instead.  The old
names are still present, but deprecated.

I'm doing this backport because it represents the deprecation of a
Libevent 2.0 interface, and folks should have the opportunity to
write code that isn't deprecated and works with both 2.0 and 2.1.