Have starter also set supplemental groups

We rely on these for resources accessed by some processes.

Change-Id: Idf22b6263ebc313c8abb63fe93bce2cc3ef24274
Signed-off-by: James Kuszmaul <james.kuszmaul@bluerivertech.com>
diff --git a/aos/starter/starterd.cc b/aos/starter/starterd.cc
index b40776d..a5a340a 100644
--- a/aos/starter/starterd.cc
+++ b/aos/starter/starterd.cc
@@ -24,6 +24,10 @@
         return 1;
       }
     }
+    // Change the real and effective IDs to the user we're running as. The
+    // effective IDs mean files we access (like shared memory) will happen as
+    // that user. The real IDs allow child processes with an different effective
+    // ID to still participate in signal sending/receiving.
     constexpr int kUnchanged = -1;
     if (setresgid(/* ruid */ gid, /* euid */ gid,
                   /* suid */ kUnchanged) != 0) {