Test unhandled exception

Commit 7fc023e8 fixed issue #64, and
made unhandled exceptions work correctly.

This patch adds a test to tst-except.cc to test this - that an unhandled
exception really does call the custom termination handler that the C++
standard allows to set (before the above commit, this test would hang).

Unfortunately, gcc's libsupc++ tries very hard to ensure that the
terminination handler really does abort - if the handler returns, or
throws an exception, the library nevertheless aborts. So we need to
trick it with an ugly longjmp to make this test be able to complete
without aborting the system.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>