Pull request: dhcpd: fix ip ranges

Updates #2541.

Squashed commit of the following:

commit c81299991876f48836d24872d9145331a0bc9e6e
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Mar 16 18:10:07 2021 +0300

    agherr: imp docs

commit f43a5f5cde0ea16dd38dd533e16e415a1d306cb2
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Mar 16 17:35:59 2021 +0300

    all: imp err handling, fix code

commit ed26ad0ff53882725f7747264f8094e6fb9b0423
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Mar 16 12:24:17 2021 +0300

    dhcpd: fix ip ranges

staticcheck.conf

@@ -1,6 +1,8 @@
 checks = ["all"]
 initialisms = [
   # See https://github.com/dominikh/go-tools/blob/master/config/config.go.
+  #
+  # Do not add "PTR" since we use "Ptr" as a suffix.
   "inherit"
 , "DHCP"
 , "DOH"
@@ -8,7 +10,6 @@ initialisms = [
 , "DOT"
 , "EDNS"
 , "MX"
-, "PTR"
 , "QUIC"
 , "RA"
 , "SDNS"