c-lib: fixed memleaks, add criu_local_free_opts()

If `criu_local_init_opts()` is applied on the same opts-object
several times, not all of the allocated memory gets freed.
Therefore, the functions `criu_(local_)free_opts()` were introduced.
These functions ensure, that opts get freed accordingly.
Furthermore, `criu_(local_)free_opts()` gets part of the c-api,
and can therefore be called by external projects too.

Additionally, with this commit `criu_local_init_opts()` now uses
`criu_local_free_opts()`, to free the opts-parameter if it was already
initalized before.

This commit also contains a fix in `send_req_and_recv_resp_sk()` which
lead to a memory leak, if criu-notifications were received.

Signed-off-by: Martin Wührer <martin.wuehrer@artech.at>
Signed-off-by: Andrei Vagin <avagin@gmail.com>