Wrap lt_dlhandle in struct m4_module.

Wrap lt_dlhandle in struct m4_module.
* m4/m4module.h (includes): No longer require clients to know
about lt_dlhandle.
(m4_module): New opaque declaration.
(m4_export): Delete unused type.
(M4INIT_HANDLER, M4FINISH_HANDLER, m4_module_init_func)
(m4_module_finish_func, m4_module_load, m4_get_module_name)
(m4_builtin_find_by_name, m4_builtin_find_by_func): Use new
wrapper.
(m4_module_makeresident, m4_module_refcount): New declarations.
* m4/m4private.h (includes): Move <ltdl.h> here.
(m4_module): New declaration.  Simple for now, but intended for
growth.
(m4__module_open, m4__module_next, m4__module_find)
(m4__symtab_remove_module_references): Use new wrapper.
* m4/builtin.c (m4_builtin_find_by_name, m4_builtin_find_by_func):
Likewise.
* m4/module.c (m4_get_module_name, m4_module_import)
(m4_module_load): Likewise.
(install_builtin_table, install_macro_table): Likewise.  Also,
change return type since these always returned NULL and no one
used the result.
(m4_module_makereside