modules: only load modules with system module file extension.

* m4/path.c (m4_load_filename): Compare with LT_MODULE_EXT instead
of hardcoded '.so'.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
modules: clean up no-longer supported preloading references.

* doc/m4.texi: Remove references to preloaded modules.
* configure.ac (Preloaded module configuration): Remove.
* Makefile.am (src_m4_LDFLAGS): Remove.  DLPREOPEN no longer set.
(src_m4_DEPENDENCIES): Remove PREOPEN_DEPENDENCIES reference.
* tests/atlocal.in (DYNAMIC_MODULES): Remove.
* tests/testsuite.at (AT_CHECK_DYNAMIC_MODULE): Remove.
* tests/builtins.at, tests/modules.at: Remove references to
removed AT_CHECK_DYNAMIC_MODULE.
* tests/generate.awk: Don't generate AT_CHECK_DYNAMIC_MODULE
calls.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
configury: improve dlsym underscore detection.

* configure.ac (fn_module_cmds): Split ~ delimited command lists,
and set globals required by $module_cmds and $archive_cmds.
(libname): Preload with 'conftmod' ready for fun_module_cmds
calls, and for reuse as an 8.3 compatible module basename in this
check.
(sys_dlsym_uscore): Use it to detect underscore prefix requirement
in some dlysm() implementations more reliably.
Reported by KO Myung-Hun

Signed-off-by: Gary V. Vaughan <gary@gnu.org>