Report proper errors from the loadlibrary loader.

Report proper errors from the loadlibrary loader.

* libltdl/loaders/loadlibrary.c (loadlibraryerror): New
helper function that returns the latest Windows error as a
string, or the provided default string on failure to do so.
(LOADLIB_SETERROR): New macro that wraps previous to make it
easy to use.
(vm_open, vm_close, vm_sym): Make use of previous.
(LOCALFREE): New macro to help free the Windows error string.
(vl_exit): Make use of previous.
* tests/loadlibarry.at: New file, new test that makes sure
the loadlibrary loader reports non-standard error messages.
* Makefile.am (TESTSUITE_AT): Add above test.

Signed-off-by: Peter Rosin <peda@lysator.liu.se>