cmdline: make struct rdline opaque

cmdline: make struct rdline opaque

Hide struct rdline definition and some RDLINE_* constants in order
to be able to change internal buffer sizes transparently to the user.
Add new functions:

* rdline_new(): allocate and initialize struct rdline.
  This function replaces rdline_init() and takes an extra parameter:
  opaque user data for the callbacks.
* rdline_free(): deallocate struct rdline.
* rdline_get_history_buffer_size(): for use in tests.
* rdline_get_opaque(): to obtain user data in callback functions.

Remove rdline_init() function from library headers and export list,
because using it requires the knowledge of sizeof(struct rdline).

Signed-off-by: Dmitry Kozlyuk <dmitry.kozliuk@gmail.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Narcisa Vasile <navasile@linux.microsoft.com>