Utils to support errgroup pattern.

Extracted from the httpsrv package as these functions are not httpsrv specific but general "errgroup pattern" helpers.

Simply put "errgroup pattern" is a organization of the service code so that all subprocesses of the service are launched as members of the same errgroup and their lifetime is controlled by the context of the group. This means that when one of the group members exits with error the group's context gets cancelled and all other group members get signal to (gracefully) exit too. Group reports the first error as the reason service stopped.

Rules for a func starting a subprocess are:

• every group member lifetime is controlled by group's context - when it gets cancelled the subprocess should exit (gracefully) ASAP;
• subprocess always returns non-nil error (most of the time it would be ctx.Err() ie the subprocess exits because the context controlling it's lifetime has been cancelled).

See the httpsrv example project for more.