Update codecov.io behavior

Update the codecov.yml included in the repository to behave as
originally intended.  This can be refined as needed.

* Always post coverage results to the GitHub PR after two builds
  have been uploaded.  This is the normal case since there will
  be a build uploaded for both kernel and user coverage results.

* Adjust red -> yellow -> green coloring in the web interface.
  Due to the number of unlikely error conditions which are hard
  to force consider 90% coverage an excellent level of coverage.

* Allow a 1% variance in coverage between test runs.  This is
  approximately 10x larger than the typical variance observed
  which leaves us a reasonable margin to prevent false positives.

* Always post a new smaller comment to PRs which does not include
  a file list.  Old coverage reports are removed.

Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #6765

.github/codecov.yml

@@ -1,30 +1,21 @@
 codecov:
   notify:
-    require_ci_to_pass: no
+    require_ci_to_pass: false	# always post
+    after_n_builds: 2		# user and kernel
 
 coverage:
-  precision: 2
+  precision: 2			# 2 digits of precision
-  round: down
+  range: "50...90"		# red -> yellow -> green
-  range: "50...100"
 
   status:
     project:
       default:
-        threshold: 1%
+        threshold: 1%		# allow 1% coverage variance
 
     patch:
       default:
-        threshold: 1%
+        threshold: 1%		# allow 1% coverage variance
-
-parsers:
-  gcov:
-    branch_detection:
-      conditional: yes
-      loop: yes
-      method: no
-      macro: no
 
 comment:
-  layout: "header, sunburst, diff"
+  layout: "reach, diff, flags, footer"
-  behavior: default
+  behavior: "new"		# delete old, post new
-  require_changes: no