- Added uinet_shutdown(), which performs libuinet cleanup when called.

- Added uinet_install_sighandlers(), which installs a cleanup handler
  on all signals whose default handler terminates the process and
  whose current handler is the default handler.  Applications that
  implement their own termination signal handlers should call
  uinet_shutdown() from those handlers.
- The netmap interface driver now restores the buffer indices in the
  receive ring during detach, to work around the netmap
  double-free/leak issue that can occur when doing zero-copy receive.
  Calling uinet_shutdown() runs the detach routines, so this netmap
  issue is worked around in all cases where uinet_shutdown() is called
  (so there is still some exposure to this via SIGKILL and when
  working in the debugger).

- Fenced off the not-ready portions of the netmap and pcap interface
  detach routines.
Updated sample programs to install the libuinet signal handlers and
use uinet_shutdown() to clean up when done.