Use syslog service in dhclient(8).

dhclient(8) is failing during boot to connect to the syslog service, because
syslog daemon is started after dhclient(8). This can be reproduced by stooping
syslog daemon and ktrace the dhclient or use kern.trap_enotcap sysctl and boot
the machine. Using the Casper syslog service fix the problem.

Reviewed by: bapt@
Differential Revision: https://reviews.freebsd.org/D12825