Define "state directory" for storing persistent child-writable state,

with default from config.layout, configurable via DefaultStateDir.

* server/core.c (set_state_dir, ap_state_dir_relative):
  New functions.
  
* config.layout, acinclude.m4, Makefile.in, configure.in: Define
  statedir variables, drop davlockdb.

* include/ap_config_layout.h.in: Define DEFAULT_REL_STATEDIR,
  DEFAULT_EXP_STATEDIR in place of _DAVLOCKDB.

* include/ap_mmn.h: Bump MMN minor.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1842929 13f79535-47bb-0310-9956-ffa450edef68