Pull request: all: imp dhcp host normalization, validation

Updates #2952.

Squashed commit of the following:

commit 45afcab5d33c1ec8176c0ad05423288c8770b772
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Apr 20 15:02:34 2021 +0300

    all: imp docs

commit d844ce1e2bb0b92a892119161774ec95f3e59711
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Apr 20 14:57:49 2021 +0300

    all: more code imp

commit eef08cb69015ead0ffaef866e12a2c6556786d37
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Apr 20 14:52:33 2021 +0300

    all: imp code, docs

commit 20748f20ab9fcc410b11daf0e23eaf156dcdc7cd
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Apr 20 14:30:57 2021 +0300

    all: imp dhcp host normalization, validation

internal/dhcpd/server.go

@@ -64,9 +64,12 @@ type V4ServerConf struct {
 
 	leaseTime  time.Duration // the time during which a dynamic lease is considered valid
 	dnsIPAddrs []net.IP      // IPv4 addresses to return to DHCP clients as DNS server addresses
-	routerIP   net.IP        // value for Option Router
-	subnetMask net.IPMask    // value for Option SubnetMask
-	options    []dhcpOption
+
+	// subnet contains the DHCP server's subnet.  The IP is the IP of the
+	// gateway.
+	subnet *net.IPNet
+
+	options []dhcpOption
 
 	// notify is a way to signal to other components that leases have
 	// change.  notify must be called outside of locked sections, since the