Tools: make unshare behave like other perl syscalls

Most syscall wrappers in perl return 1 on success and our
current use of Tools::unshare isn't using the return value
(yet), so let's fix this while we can.

Also it seems to make sense to use prototyping on syscalls
to add some compile-time argument checking.

src/PVE/Tools.pm

@@ -1169,9 +1169,9 @@ sub parse_host_and_port {
     return; # nothing
 }
 
-sub unshare {
+sub unshare($) {
     my ($flags) = @_;
-    syscall 272, $flags;
+    return 0 == syscall(272, $flags);
 }
 
 1;