dhcpleased(8) - DHCP client daemon

With the following commit, Florian Obser (florian@) imported dhcpleased(8), DHCP daemon to acquire IPv4 address leases from servers, plus dhcpleasectl(8), a utility to control the daemon:

CVSROOT:	/cvs
Module name:	src
Changes by:	florian@cvs.openbsd.org	2021/02/26 09:16:37

Added files:
	sbin/dhcpleased: Makefile bpf.c bpf.h checksum.c checksum.h 
	                 control.c control.h dhcpleased.8 dhcpleased.c 
	                 dhcpleased.h engine.c engine.h frontend.c 
	                 frontend.h log.c log.h 
	usr.sbin/dhcpleasectl: Makefile dhcpleasectl.8 dhcpleasectl.c 
	                       parser.c parser.h 

Log message:
Import dhcpleased(8) - a dhcp daemon to acquire IPv4 address leases
from servers.

dhcpleased(8) follows the well known three process design of all our
privsep daemons. It uses pledge(2) and unveil(2) to restrict access
further. In particular the "engine" process, responsible for parsing
of untrusted data, is pledge'd "stdio". It cannot access the outside
world nor the filesystem at all.

Like slaacd(8) for IPv6 it will be always running and acquire addresses
for all interface with the autoconf4 flag set.
The flag can be set by "ifconfig $if inet autoconf" or by adding
"inet autoconf" to /etc/hostname.if. An existing "dhcp" line should
be removed.

Various iterations tested by deraadt@
The hardest part, finding a name, was handled by jmatthew@ & otto@

"get to it :)" deraadt@

The new daemon is (linked to the build and) enabled by default.