Pull request: dhcpd: do not override ra-slaac settings

Merge in DNS/adguard-home from 2653-ra-slaac to master

Updates #2653.

Squashed commit of the following:

commit f261413a58dc813e37cc848606ed490b8c0ac9f3
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Feb 11 20:37:13 2021 +0300

    all: doc changes, rm debug

commit 4a8c6e4897579493c1ca242fb8f0f440c3b51a74
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Feb 11 20:11:46 2021 +0300

    dhcpd: do not override ra-slaac settings

staticcheck.conf

@@ -10,7 +10,9 @@ initialisms = [
 , "MX"
 , "PTR"
 , "QUIC"
+, "RA"
 , "SDNS"
+, "SLAAC"
 , "SVCB"
 ]
 dot_import_whitelist = []