Redo the config.h / event-config.h / evconfig-private.h arrangement.

All .c files in libevent now include the original, autoconf-generated
config.h as their very first act, and use the unmangled macro names
for all configuration parameters.  All internal header files assume
config.h has already been included, and use the unmangled macro names.

The public headers still include event-config.h, which is still
generated from config.h and still exposes mangled names of the form
EVENT__whatever (also the special LIBEVENT_VERSION and
LIBEVENT_VERSION_NUMBER macros, which are now directly provided by
event-config.h).  However, the generation process for this file now
prunes all macros which are not actually mentioned in any public
header.

The macros defined by AC_USE_SYSTEM_EXTENSIONS and AC_SYS_LARGEFILE
now no longer require special handling, so evconfig-private.h has been
removed.  Some more .c files that had a surrounding #ifdef and were
also conditionally compiled by the Makefile have had the #ifdef
removed.  Finally, the non-autotools Windows build machinery, which
appears to have been out of date before I started changing things, was
also removed.  (No reason it couldn't come back, if someone actually
keeps it updated.)