syslogd: Enter capability mode

Create syslogd_cap_enter() function, which puts syslogd in capability
mode and opens necessary libcasper channels for resource acquisition.
The syslogd.casper and system.net services are used.

Substitute capability-mode-unfriendly functions with their libcasper
cap_ equivalents.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D41467