[multipathd] add CLI readline completion

[multipathd] add CLI readline completion

This one needed a bit of work :

o Add a convenient vector_foreach_slot_after() to the vector lib
o Move all add_handler() calls from main.c to cli.c, but don't set handlers
  there. This way the multipathd binary execed as the CLI (-k flag) gains
  the leisure the load the CLI dictionnary too. Which is quite useful for
  keyword completion.
o Add a set_handler_callback() function and use it in main.c in place of
  former add_handler() calls.
o No need to compute len each vector_foreach_slot() iteration in find_key()
o get_cmdvec() returns more precise failure hints as int return value.
  The readline keyword generator needs those hints.