This lets the 'childhurd' service start in the background, letting
shepherd perform other tasks in the meantime, including serving
clients (such as the 'herd' command).
* gnu/build/secret-service.scm (with-modules): New macro.
(wait-for-readable-fd): Add cooperative implementation when Fibers is in
use.
(secret-service-send-secrets): Define 'sleep' so that it cooperates when
Fibers is in use.