Implement lt_dlopening of only preloaded modules.

Implement lt_dlopening of only preloaded modules.
* libltdl/m4/ltdl.m4 (LTDL_INIT): Check for a libltdl that
provides lt_dladvise_preopen when deciding if installed libltdl
is 'new enough'.
* libltdl/libltdl/lt__private.h (lt__advise): Add a new
is_preload flag.
* libltdl/ltdl.c (lt_dladvise_preload): New api call to set it.
(try_dlopen): If it is set, and the search of preloaded modules
didn't return a match, don't bother searching the filesystem.
* libltdl/ltdl.h (lt_dladvise_preload): Declare it.
* doc/libtool.texi (Libltdl Interface): Document it.
* tests/lt_dladvise.at: Test it (and incidentally add some test
coverage for `libtool -dlpreopen').
* NEWS: Announce it.