Don't use deprecated libevent code

This pull request modifies the Poller implementation not to use deprecated libevent code, as promised during the review of #11.

Working on that, I also realized that the `SIGINT` handler should not be installed when the Poller is created, for two reasons. First, we don't need such handler on Android, we only need it for test programs. Second, in Libevent there could be only one SIGINT handler, so it's incorrect to register such handler every time we create a Poller (or we loop). Yes, we plan to use a single Poller, but it's even better to write correct code that works correctly also when multiple Pollers are instantiated.

To test this pull request, I verified that no new warnings are introduced and I run the `echo_client` and `echo_client_evbuf` tests.
Of the possible ways to avoid `goto` that we discussed yesterday, I chose the one that adds less lines of code, i.e., I freed in each error path the things that were allocated so far. A patch for that is now sitting on top of the current branch.