Rename ".i" files in source tree.

There are four .i files in the WiredTiger source tree.  Two are specialized C include files, and probably need to be renamed to something like x_inline.h Two are SWIG interface files, and would probably be clearer if they were named x.swig .  By fixing this, our tooling and scripts in the dist directory would more easily include the right files.  This change probably will require us to fix the formatting in the two C include files as part of the ticket.