ATS 9.2.6 fails to start as root

[jvgutierrez]

As a side effect of #11855 ATS is now unable to start as root:

traffic_manager uses ImpersonateUser() to drop privileges triggering the following syscalls:

450941 setgroups(1, [123])              = 0
450941 setresgid(-1, 123, -1)           = 0
450941 setresuid(-1, 118, -1)           = 0

afterwards traffic_server attempts to drop privileges as well but setgroups fails due to lack of permissions:

450952 setgroups(1, [123])              = -1 EPERM (Operation not permitted)

[jvgutierrez]

-  if (initgroups(pwd->pw_name, pwd->pw_gid) != 0) {
-    Fatal("switching to user %s, failed to initialize supplementary groups ID %ld", pwd->pw_name, (long)pwd->pw_gid);
+  if (geteuid() == 0) {
+    if (initgroups(pwd->pw_name, pwd->pw_gid) != 0) {
+      Fatal("switching to user %s, failed to initialize supplementary groups ID %ld", pwd->pw_name, (long)pwd->pw_gid);
+    }

could be a potential solution for this, @maskit what do you think?

[maskit]

Oh, traffic_manager... Yeah, it's an oversight and there was no intention to make ATS unable to start as root. Would you make a PR?