Rationalize backend selection and make it controllable by configure options.

* libevent_arg.m4: Add new macros LIBEVENT_ENABLE_BACKENDS,
  LIBEVENT_BACKEND_PREP (internal), LIBEVENT_BACKEND,
  LIBEVENT_FINALIZE_BACKENDS.  Factor out a common m4-level name conversion.

* configure.ac: Move AM_INIT_AUTOMAKE below all the --enable options;
  some of them can now bomb out the configure run, and we want that to
  happen before any slow checks if possible.  Add
  LIBEVENT_ENABLE_BACKENDS to the --enable options block.  Redo the
  backend-selection block using LIBEVENT_BACKEND and
  LIBEVENT_FINALIZE_BACKENDS; move it below library selection; merge
  AC_CHECK_FUNCS calls hiding in the backend-selection block into the
  main one.

* event.c: Backends now selected with consistent EVENT__BACKEND_* macros.
* Makefile.am: *_BACKEND conditionals are now all BACKEND_* for
  consistency with the preprocessor macros.
* libevent_func_checks.m4: Rename 'haveepoll' to 'libevent_have_epoll'.
* devpoll.c, epoll.c, evport.c: Remove #ifdef around entire file; this
  is redundant to Makefile machinery.
* test/regress.c: Suppress [warn] messages from main/methods and
  main/base_environ when all backends are disabled.