MFC r225849:

Test if the interface is afif in dhcpif() and syncdhcpif(), as
done in ipv6_autoconfif.

This fixes a regression that causes e.g. ifconfig_DEFAULT="DHCP"
to run on non-afif interfaces like pfsync0, which in turn would
cause excessive delay on system startup.

Sponsored by: iXsystems, Inc.
Reviewed by: hrs (freebsd-rc@)
Approved by: re (bz)