include/Makefile.mk,include/xapian/enquire.h,
include/xapian/iterator.h: Add simple API to help with creating
language-idiomatic iterator wrappers.

git-svn-id: file:///home/xapian-svn/xapian/trunk@18250 6b2f1b92-a4f3-0310-943f-de3cbe41748b