Add "find_new_uid" script.

This script automates the process of finding a new UID to use by examing
all currently-in-use UIDs.

BUG=None
TEST=Manually ran script and verified that output was correct.

Change-Id: If09aff0b4f3c01a30d8de949a3caa35600ece63f
Reviewed-on: https://chromium-review.googlesource.com/1600372
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Miriam Zimmerman <mutexlox@chromium.org>
Reviewed-by: LaMont Jones <lamontjones@chromium.org>

profiles/base/accounts/README.md

diff --git a/profiles/base/accounts/README.md b/profiles/base/accounts/README.md
index a592cd5..efd623d 100644
--- a/profiles/base/accounts/README.md
+++ b/profiles/base/accounts/README.md
@@ -52,7 +52,8 @@
 CrOS system daemon UIDs (and associated GIDs) range from 200-299 and
 from 20100-29999. If you're creating a new user, pick the first UID in
 the range 20100-29999 that is not currently used, and create both a user
-and a group with this ID.
+and a group with this ID.  To find the next available UID, invoke
+`./find_new_uid.sh`.
 
 FUSE-based filesystem daemons have UID/GIDs that range from 300-399.
 If you're adding a daemon that will be talking to `cros-disks` and

profiles/base/accounts/find_new_uid.sh

diff --git a/profiles/base/accounts/find_new_uid.sh b/profiles/base/accounts/find_new_uid.sh
new file mode 100755
index 0000000..8fd0f0a
--- /dev/null
+++ b/profiles/base/accounts/find_new_uid.sh
@@ -0,0 +1,9 @@
+#!/bin/bash
+set -euo pipefail
+
+files="$(dirname $0)/user/*"
+echo -n "Your new UID is: "
+awk --field-separator : '
+  BEGIN { max = 20099 }
+  /^uid:/ { if ($2 <= 29999 && $2 > max) max = $2 }
+  END { print max + 1}' $files