net/dhcpcd: Use HAS_CONFIGURE over GNU_CONFIGURE

Historically the port used GNU_CONFIGURE but this is not needed.
Switch to HAS_CONFIGURE and adjust the needed configure args.
While here install the sample config file under etc/.

Reported by:		diizzy
Approved by:		0mp (mentor)
Differential Revision:	https://reviews.freebsd.org/D32907