Centralize root check, user ansible var, not env var

fixes #17 - the current check logic was failing under docker which
doesnt always inject a `USER` environment variable. Rather than try to
work around that in docker, lets update our root check logic to be more
universally compatible.

tasks/become_check.yml

@@ -0,0 +1,13 @@
+---
+- name: Run setup as same user as role
+  setup:
+
+- name: Stop if running with elevated privileges.
+  fail:
+    msg: >-
+      WARNING! You should not compile the Linux kernel with
+      superuser privileges. Doing so may create an unbootable
+      system. See Greg Kroah-Hartman's Linux Kernel in a Nutshell
+      for anecdotes about how doing so has broken systems
+      in the past: http://www.kroah.com/lkn/
+  when: ansible_user_id == 'root'

tasks/main-stable.yml

@@ -1,14 +1,4 @@
 ---
-- name: Stop if running with elevated privileges.
-  fail:
-    msg: >-
-      WARNING! You should not compile the Linux kernel with
-      superuser privileges. Doing so may create an unbootable
-      system. See Greg Kroah-Hartman's Linux Kernel in a Nutshell
-      for anecdotes about how doing so has broken systems
-      in the past: http://www.kroah.com/lkn/
-  when: ansible_env.USER == 'root'
-
   # Install packages before fetching dynamic URLs, since python-requests
   # is required by the URL-fetching Ansible module.
 - include: packages.yml

tasks/main-unofficial.yml

@@ -1,14 +1,4 @@
 ---
-- name: Stop if running with elevated privileges.
-  fail:
-    msg: >-
-      WARNING! You should not compile the Linux kernel with
-      superuser privileges. Doing so may create an unbootable
-      system. See Greg Kroah-Hartman's Linux Kernel in a Nutshell
-      for anecdotes about how doing so has broken systems
-      in the past: http://www.kroah.com/lkn/
-  when: ansible_env.USER == 'root'
-
 - name: Install stretch backports repository.
   become: yes
   apt_repository:

tasks/main.yml

@@ -1,4 +1,6 @@
 ---
+- include: become_check.yml
+
 - include: main-stable.yml
   when: grsecurity_build_patch_type != 'unofficial'