Add `dir' file to list the info manual

If a user doesn't want to install geiser, it's possible to run it in
place by loading `elisp/geiser.el' directly.

It's also possible to read the info manual directly but it's
inconvenient to not have it listed when hitting C-h i inside Emacs.

To list such manual, it's necessary to tell info where to look for it.
One way of doing it is adding the directory to the variable
`Info-additional-directory-list'.  But, for this to work, there's need
to be a `dir' file listing the info manual.
Add `dir' to EXTRA_DIST

So the generated tarball from `make dist' will have `dir' inside.
A file not ending in newline, oh the horror
Newish emacsen don't have comint-last-prompt-overlay

... so we have to check if it's defined and, in its defect, use the
new shiny comint-last-prompt.  A bit of refactoring resulted.