This commit offers three main features:

  * It improves the target argp parsing functions so that the command
    lines of Stackdb programs can be used to specify either a single
    target; OR both base and overlay targets.
  * It improves the target instantiation functions so that multiple,
    potentially dependent targets can all be instantiated and opened,
    without the user having to manage the dependency ordering.
  * It adds target_monitor_evloop, so that a user can easily, safely,
    and synchronously monitor multiple targets (as long as all their
    drivers support evloops -- all drivers currently do).
  * It adds much default signal handling functionality, via
    target_install_default_sighandlers, etc.  Now users don't have
    to explicitly handle signals --- and they can use the target
    library's safe signal support to handle the bits they care about
    (i.e., SIGHUP to reload a config file; SIGALRM to do time-bas...