[new API] two-phased qb_ipcs_create required

Signatures
```
/* Creates a listening UNIX socket, akin to qb_ipcs_create */
qb_ipcs_listener_t* qb_ipcs_listener(const char *name);

/* Gets socket descriptor from listener; to prevent further
use (the same information is now at two places at minimum),
deallocates l, so _it must not be reused elsewhere from that
point on_ */
int qb_ipcs_listener_fd(const qb_ipcs_listener_t *l);

/* Wraps a listening UNIX socket, presumably created with
qb_ipcs_listener and subsequently extracted with qb_ipcs_listener_fd
(e.g. the name gets validated), extracting "name" using
getsockname(2); _fd must not be reused elsewhere from that point on_ */
qb_ipcs_listener_t* qb_ipcs_listener_from_fd(int fd);

/* Reuses existing UNIX socket wrapped in a listener object;
qb_ipcs_listener + qb_ipcs_create_from_listener equals qb_ipcs_create;
to prevent further use, deallocates l, so _it must not be reused
elsewhere from that point on_ */
qb_ipcs_service_t* qb_ipcs_create_from_listener(qb_ipcs_listener_t *l,
int32_t service_id,
enum qb_ipc_type type,
struct qb_ipcs_service_handlers *handlers);
```

Allow safe nesting wrt. NULL checking, e.g.,

```
qb_ipcs_create_from_listener(qb_ipcs_listener_from_fd(qb_ipcs_listener_fd(qb_ipcs_listener("foo"))),
42, QB_IPC_SHM, handlers);
```

The `qb_ipcs_listener_t` abstraction is meant to allow for (and enforce)
extra checking on libqb side (so that the same internal coherency as with
monolithic `qb_ipcs_create` is achieved), as that type is only used as
an opaque pointer in the client program.