Add loadable modules using Daniel Colascione's ideas.

Add loadable modules using Daniel Colascione's ideas.

See https://lists.gnu.org/archive/html/emacs-devel/2015-02/msg00960.html

* src/Makefile.in (base_obj): add module.o
  (LIBES): add -lltdl

* src/emacs.c (main): add syms_of_module

* src/lisp.h: add syms_of_module

* src/emacs_module.h: New file; emacs API for modules.

* src/module.c: New file; implement API.

* modules/basic/Makefile: New file; build example module on Linux.

* modules/basic/basic.c: New file; simple example module.