The concept of managed closure is officially extinct.

- Remove the launch screen.
- Add a label explaining where to look for the code and results.
- Delete unnecessary boilerplate code.

The only remaining need for those (whereby the closure frees itself upon
completion) remains in security/credentials.c, where the code has been
inlined (to be removed at a later time).

alarm_test, tcp_posix, fd_posix, pollset_posix, credentials, call,
channel, server, child_channel

The tests are going to use it too

In library projects (cf. app projects) the main bundle is nil. `NSBundle+bundleForClass:` works in both types of projects.

Also makes the library load the certificates only once.

Because the sample protos were all in a flat directory, I hadn't noticed the issue.

Mostly to facilitate testing, but maybe in the future we want to work on
Linux systems with broken epoll support?