Pull request: all: openbsd support

Updates #2439.

Squashed commit of the following:

commit 3ff109e43751132d77500256c8869938680ac281
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Jun 3 20:46:17 2021 +0300

    all: imp code, docs

commit 512ee6d78cfee511f429d09c8366bb7dd8019aa8
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Jun 3 20:06:41 2021 +0300

    all: openbsd support

internal/aghos/os.go

@@ -5,18 +5,28 @@ import (
 	"fmt"
 	"os/exec"
 	"syscall"
+
+	"github.com/AdguardTeam/golibs/errors"
 )
 
+// ErrUnsupported is returned when the functionality is unsupported on the
+// current operating system.
+//
+// TODO(a.garipov): Make a structured error and use it everywhere instead of
+// a bunch of fmt.Errorf and all that.
+const ErrUnsupported errors.Error = "unsupported"
+
 // CanBindPrivilegedPorts checks if current process can bind to privileged
 // ports.
 func CanBindPrivilegedPorts() (can bool, err error) {
 	return canBindPrivilegedPorts()
 }
 
-// SetRlimit sets user-specified limit of how many fd's we can use
-// https://github.com/AdguardTeam/AdGuardHome/internal/issues/659.
-func SetRlimit(val uint) {
-	setRlimit(val)
+// SetRlimit sets user-specified limit of how many fd's we can use.
+//
+// See https://github.com/AdguardTeam/AdGuardHome/internal/issues/659.
+func SetRlimit(val uint64) (err error) {
+	return setRlimit(val)
 }
 
 // HaveAdminRights checks if the current user has root (administrator) rights.