Replace libltdl with raw dlopen, dlsym

* NEWS: Update.
* am/bootstrap.am (SOURCES):
* module/Makefile.am (SOURCES): Add system/foreign-library.scm.
* configure.ac: Replace ltdl check with -ldl check.
* libguile/dynl.c: Rewrite to just expose core dlopen / dlsym / etc to a
  helper Scheme module.
  (scm_dynamic_link, scm_dynamic_pointer, scm_dynamic_function)
  (scm_dynamic_object_p, scm_dynamic_call): Rewrite in terms of (system
  foreign-library).
* libguile/extensions.c (load_extension): Avoid scm_dynamic_call.
* module/system/foreign-library.scm: New file.
* module/oop/goops.scm (<dynamic-object>): Hackily export
  <foreign-library> instead of a class here.
* doc/ref/api-foreign.texi (Foreign Function Interface): Rewrite to only
  document the new interfaces.  Eventually we will deprecate
  dynamic-link and friends.
* doc/ref/guile.texi (API Reference): Move Foreign Objects after Foreign
  Function Interface.  Seems there should be some closer relationship
  but this will do for now.
* doc/ref/tour.texi (Putting Extensions into Modules):
* doc/ref/libguile-parallel.texi (Parallel Installations): Update for
  rename of Modules and Extensions to Foreign Extensions.
* libguile/deprecated.h:
* libguile/deprecated.c (scm_dynamic_unlink): Deprecate.
* libguile/guile.c: Remove ltdl include.
* test-suite/tests/foreign.test: Update tests to use new API, and update
  error expectations.

am/bootstrap.am

@@ -1,4 +1,4 @@
-##  	Copyright (C) 2009-2020 Free Software Foundation, Inc.
+##  	Copyright (C) 2009-2021 Free Software Foundation, Inc.
 ##
 ##   This file is part of GNU Guile.
 ##
@@ -120,6 +120,7 @@ SOURCES =					\
   system/vm/program.scm				\
   system/vm/vm.scm				\
   system/foreign.scm				\
+  system/foreign-library.scm			\
 						\
   language/tree-il/compile-cps.scm		\
   language/tree-il/cps-primitives.scm		\