Add #ifndef wrappers to src/main/*.h files to insulate them

	against multiple inclusions.  httpd.h now includes ap.h for
	the ap_*() routines.

Reviewed by:	Commit-then-review rules


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@79928 13f79535-47bb-0310-9956-ffa450edef68