syslogd: Use pipe to communicate with daemon

Previously, syslogd's daemon process would signal that it had finished
initialization using SIGALRM. In capability mode, signal delivery is not
allowed, so use a pipe to indicate that it is ready to accept messages.

Reviewed by: markj
MFC after: 3 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D41459