Pull request: 3172 fix mobileconfig

Merge in DNS/adguard-home from 3172-mobileconfig to master

Updates #3172.
Updates #2497.

Squashed commit of the following:

commit 30549ef4eda9d88f0738089e901492d7369caa25
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Tue Jun 1 21:00:17 2021 +0300

    all: log changes

commit 9b9429447430a8e5656b992c04c4a74606dc5f9f
Author: Ildar Kamalov <ik@adguard.com>
Date:   Tue Jun 1 17:56:59 2021 +0300

    client: always show port input

commit 6d6a0bdfaa849220a5ddb4a17502ab05379d7a1c
Merge: 13a3bffd 77946a7f
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Tue Jun 1 17:50:41 2021 +0300

    Merge branch 'master' into 3172-mobileconfig

commit 13a3bffd4dd6ccabf3d261f17b2c758a5c61eb9c
Author: Ildar Kamalov <ik@adguard.com>
Date:   Tue Jun 1 17:20:17 2021 +0300

    client: add port to mobile config form

commit f6abe0b6044572f3801c31b683e76f90c4a28487
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Mon May 31 19:43:37 2021 +0300

    home: imp cyclo

commit c304a0bacdca6f8b5ffd21f3d00c8244ea9e4e36
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Mon May 31 18:19:46 2021 +0300

    home: reduce allocs

commit 10a7678861079b710bb0ef14569c60a09612ec70
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Mon May 24 20:05:08 2021 +0300

    all: make the host parameter required

internal/aghstrings/strings.go

@@ -21,7 +21,8 @@ func CloneSlice(a []string) (b []string) {
 
 // Coalesce returns the first non-empty string.  It is named after the function
 // COALESCE in SQL except that since strings in Go are non-nullable, it uses an
-// empty string as a NULL value.  If strs is empty, it returns an empty string.
+// empty string as a NULL value.  If strs or all it's elements are empty, it
+// returns an empty string.
 func Coalesce(strs ...string) (res string) {
 	for _, s := range strs {
 		if s != "" {