* Split libyaml (C library) and lua-yaml (Lua bindings)
* Add support for system libyaml-dev in CMake scripts if
   -DENABLE_BUNDLED_LIBYAML is OFF (default is ON)
* Add libyaml-dev to DEB dependencies

third_party/libyaml/ is completely identical to upstream libyaml 0.1.6.
(libyaml-0-2 in Debian/Ubuntu). Test are passed both with bundled and
system versions of the library.

third_party/lua-yaml/ is patched by me to use luaL_serializer, therefore
upstream version is not suitable for Tarantool.

This patch is needed to pass Debian package checks (lintian).