parse-options: make some arguments optional, add callbacks.

parse-options: make some arguments optional, add callbacks.

* add the possibility to use callbacks to parse some options, this can
  help implementing new options kinds with great flexibility. struct option
  gains a callback pointer and a `defval' where callbacks user can put
  either integers or pointers. callbacks also can use the `value' pointer
  for anything, preferably to the pointer to the final storage for the value
  though.

* add a `flag' member to struct option to make explicit that this option may
  have an optional argument. The semantics depends on the option type. For
  INTEGERS, it means that if the switch is not used in its
  --long-form=<value> form, and that there is no token after it or that the
  token does not starts with a digit, then it's assumed that the switch has
  no argument. For STRING or CALLBACK it works the same, except that the
  condition is that the next atom starts with a dash. This is needed to
  implement backward compatible behaviour with existing ways to parse the
  command line. Its use for new options is discouraged.

Signed-off-by: Pierre Habouzit <madcoder@debian.org>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>