Replace big chain of if/thens in epoll.c with a table lookup

This should save a bunch of branches by doing instead a lookup in a
nice static table.

To ensure correctness, the table is generated from a Python script,
included with this commit.
Clean up error handling in epoll_apply_one_change() a little

The old code was more or less:
   if (op == X && errno == FOO) {
     ...
   } else if (op == Y && errno == BAR) {
     ...
   }
but really we wanted to do a switch (op) to avoid needless checks
and branches.

This patch leaves the indentation a little weird so as to make it
easier to see what changed; the next patch will fix the indentation.
Reindent epoll_apply_one_change()
Merge branch 'epoll_table_v2'
Add evbuffer_add_file_segment() so one fd can be used efficiently in more than one evbuffer_add_file at a time
Add support for mmaps with nonzero offset values.  Needs testing.
Add CreateFileMapping file_segment implementation for win32
Refactor addfile tests; add test for big files and for offsets
Merge branch 'file_offset_v2'
Add AC_USE_SYSTEM_EXTENSIONS to configure.in.
Requires follow on patches for correctness and robustness.
Filter '# define' statements from autoconf and generate event-private.h
Add event-private.h to noinst_HEADERS
Remove internal usage of _GNU_SOURCE
Eliminate a couple more manual internal _GNU_SOURCE defines
Fix evport handling of POLLHUP and POLLERR

In other backends, they make _all_ events trigger; with evport they
previously triggered nothing.  Found by Phua Keat Yee.
Fix compilation on Windows with NDEBUG

Dongsheng Song reports that when building on windows with NDEBUG, you
run into an attempt to do EVUTIL_ASSERT(x) where x is a bitfield,
which turns into _EVUTIL_NIL_CONDITION(x), which takes sizeof(x),
which is illegal.  This patch fixes _EVUTIL_NIL_CONDITION to work on
bitfields too.
Merge remote branch 'origin/patches-2.0'
Add AC_GNU_SOURCE to the fallback case.
evconfig-private.h with recusive configure subdirs
Revert "evconfig-private.h with recusive configure subdirs"

This reverts commit ed53d5ef8053af08186ef80dfc1647cff769fac8.