Security fix for CVE-2009-1195: fix Options handling such that

'AllowOverride Options=IncludesNoExec' does not permit Includes with
exec= enabled to be configured in an .htaccess file:

* include/http_core.h: Change semantics of Includes/IncludeNoExec
  options bits to be additive; OPT_INCLUDES now means SSI is enabled
  without exec=.  OPT_INCLUDES|OPT_INC_WITH_EXEC means SSI is enabled
  with exec=.

* server/core.c (create_core_dir_config): Remove defunct OPT_INCNOEXEC
  from default override_opts; no functional change.
  (merge_core_dir_configs): Update logic to ensure that exec= is
  disabled in a context where IncludesNoexec is configured, even if
  Includes-with-exec is permitted in the inherited options set.
  (set_allow_opts, set_options): Update to reflect new semantics
  of OPT_INCLUDES, OPT_INC_WITH_EXEC.

* server/config.c: Update to remove OPT_INCNOEXEC from default 
  override_opts; no functional change.

* modules/filters/mod_include.c (includes_filter): Update to reflect
  new options sema...