Pull request: * all: move internal Go packages to internal/

Merge in DNS/adguard-home from 2234-move-to-internal to master

Squashed commit of the following:

commit d26a288cabeac86f9483fab307677b1027c78524
Author: Eugene Burkov <e.burkov@adguard.com>
Date:   Fri Oct 30 12:44:18 2020 +0300

    * all: move internal Go packages to internal/

    Closes #2234.

internal/util/os_unix.go

@@ -0,0 +1,32 @@
+// +build aix darwin dragonfly linux netbsd openbsd solaris
+
+package util
+
+import (
+	"os"
+	"syscall"
+
+	"github.com/AdguardTeam/golibs/log"
+)
+
+// Set user-specified limit of how many fd's we can use
+// https://github.com/AdguardTeam/AdGuardHome/internal/issues/659
+func SetRlimit(val uint) {
+	var rlim syscall.Rlimit
+	rlim.Max = uint64(val)
+	rlim.Cur = uint64(val)
+	err := syscall.Setrlimit(syscall.RLIMIT_NOFILE, &rlim)
+	if err != nil {
+		log.Error("Setrlimit() failed: %v", err)
+	}
+}
+
+// Check if the current user has root (administrator) rights
+func HaveAdminRights() (bool, error) {
+	return os.Getuid() == 0, nil
+}
+
+// SendProcessSignal - send signal to a process
+func SendProcessSignal(pid int, sig syscall.Signal) error {
+	return syscall.Kill(pid, sig)
+}