Factor out the individual loaders, in preparation for preloading

Factor out the individual loaders, in preparation for preloading
them as libtool modules.  Currently loader-preopen.c is linked
unconditionally and other appropriate loaders discovered by
AC_LTDL_DLLIB are built and linked in:

* libltdl/Makefile.am (libltdl_la_SOURCES): Add loader-preopen
module.
* m4/ltdl.m4 (AC_LTDL_DLLIB): Use AC_LIBOBJ to add appropriate
loaders to libltdl.
* libltdl/lt_system.h (LT_SCOPE): Moved to here.
* libltdl/ltdl.h (lt_dlloader, lt_user_data, lt_module)
(lt_module_open, lt_module_close, lt_find_sym, lt_dlloader_exit)
(struct lt_user_dlloader, lt_dlloader_next, lt_dlloader_find)
(lt_dlloader_name, lt_dlloader_data, lt_dlloader_add)
(lt_dlloader_remove): Moved declarations...
* libltdl/lt_loader.h: ...to here.
* libltdl/ltdl.c: Include lt_loader.h.  Move loader implementation
code from here...
* libltdl/loader-dld_link.c, libltdl/loader-dlopen.c,
libltdl/loader-dyld.c, libltdl/loader-load_add_on.c,
libltdl/loader-loadlibrary.c, libltdl/loader-preopen.c,
libltdl/loader-shl_load.c: ...to here.
* tests/cdemo/Makefile.am, tests/demo/Makefile.am,
tests/depdemo/Makefile.am, tests/f77demo/Makefile.am,
tests/mdemo/Makefile.am, tests/mdemo2/Makefile.am,
tests/pdemo/Makefile.am, tests/tagdemo/Makefile.am (AM_CPPFLAGS):
Add -I$(top_srcdir)/../.. so that libltdl include files can be
written as #include <libltdl/lt_dlloader.h> and found correctly.