add an <IfFile> config section like <IfDefine>

It allows a non httpd config file to be used as a marker directly in
httpd.conf without hiding logic in a script in front of apachectl
to do test -f and pass extra -D's.

This is something we've had in IBM's httpd distro for a little bit and
hadn't remembered to share. I've seen some questions/config files come
up in a few places lately that would benefit from this as an option.

Remove duplicate implementations of conditional section function.  No
functional change.

* server/core.c (start_cond_section): Factor out from start_if*.
  Adjust to use apr_strmemdup.
  (test_ifmod_section, test_iffile_section, test_ifdefine_section):
  Move container-specific tests into callbacks from start_if*.
  (core_cmds): Adjust <IfFile, <IfDefine, <IfModule to use
  start_cond_section with callbacks.

Add <IfDirective> and <IfSection>:

* server/core.c
  (test_ifdirective_section, test_ifsection_section): New callback...