cmdline: make implementation logically opaque

cmdline: make implementation logically opaque

struct cmdline exposes platform-specific members it contains, most
notably struct termios that is only available on Unix. While ABI
considerations prevent from hinding the definition on already supported
platforms, struct cmdline is considered logically opaque from now on.
Add a deprecation notice targeted at 20.11.

* Remove tests checking struct cmdline content as meaningless.

* Fix missing cmdline_free() in unit test.

* Add cmdline_get_rdline() to access history buffer indirectly.
  The new function is currently used only in tests.

Suggested-by: Olivier Matz <olivier.matz@6wind.com>
Signed-off-by: Dmitry Kozlyuk <dmitry.kozliuk@gmail.com>
Acked-by: Ray Kinsella <mdr@ashroe.eu>
Acked-by: Olivier Matz <olivier.matz@6wind.com>