Fix PR_SET_CHILD_SUBREAPER call

PR_SET_CHILD_SUBREAPER actually requires a non-zero argument to `prctl`
in order to work..!

Now, this used to work just fine (and currently works in most places)
because when we use a libc that doesn't know about
PR_SET_CHILD_SUBREAPER, it doesn't do anything about the second argument
passed to `pctrl`, so we end up sending some junk in as the second
argument. What we send appears to be completely random and as such seems
very unlikely to be zero, and so things appear to work (e.g. the tests
all pass, etc.).

However, using a libc that does know about this argument (e.g. Ubuntu
Xenial), things *don't* work because the second argument is
automatically set to 0 when we don't provide one.

This probably went unnoticed for a while considering that `tini-static`
isn't affected (it's built on Trusty), and that this mode isn't enabled
by default in the first place.

src/tini.c

@@ -306,7 +306,7 @@ int parse_env() {
 #if HAS_SUBREAPER
 int register_subreaper () {
 	if (subreaper > 0) {
-		if (prctl(PR_SET_CHILD_SUBREAPER)) {
+		if (prctl(PR_SET_CHILD_SUBREAPER, 1)) {
 			if (errno == EINVAL) {
 				PRINT_FATAL("PR_SET_CHILD_SUBREAPER is unavailable on this platform. Are you using Linux >= 3.4?")
 			} else {