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/iprange.go

@@ -110,3 +110,8 @@ func (r *ipRange) offset(ip net.IP) (offset uint64, ok bool) {
 	// construction.
 	return offsetInt.Uint64(), true
 }
+
+// String implements the fmt.Stringer interface for *ipRange.
+func (r *ipRange) String() (s string) {
+	return fmt.Sprintf("%s-%s", r.start, r.end)
+}