Clear out doc strings matching DOC file before dumping.

Clear out doc strings matching DOC file before dumping.

Since we have to call Snarf-documentation when reloading the saved
Lisp environment to get the subr doc pointers correct, we should omit
from the saved environment any doc strings that will be re-acquired
via Snarf-documentation anyway.

* src/doc.c (store_function_docstring): Add a new argument which,
if true, causes documentation slots to be set to indicate a zero
offset, which will be replaced next time Snarf-documentation is
called.
(Fsnarf_documentation): Add an optional second argument that says to
clear out as much as possible any documentation that can be found
later with a normal call to Fsnarf_documentation.
(reread_doc_file): Supply nil as the new argument to
Fsnarf_documentation.
* lisp/loadup.el: When preparing to dump the environment, call
Snarf-documentation with the extra argument to clear out the doc
strings that we can reload at startup time.