syslogd: Refresh configuration using libcasper

When a SIGHUP signal is sent to syslogd, the configuration is reparsed,
leading to new resource acquisition.

If syslogd is running in capability mode and a SIGHUP is received, new
resources cannot be acquired. To mitigate this issue, libcasper is used
to parse the configuration.

The libcasper process runs outside of capability mode and is capable of
parsing syslogd's configuration and obtaining new resources. These
resources are then sent to the syslogd process via nvlist.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D41464