HDFS-7476. Consolidate ACL-related operations to a single class. Contributed by Haohui Mai.

(cherry picked from commit 9297f980c2de8886ff970946a2513e6890cd5552)
HADOOP-11313. Adding a document about NativeLibraryChecker. Contributed by Tsuyoshi OZAWA.

(cherry picked from commit 1b3bb9e7a33716c4d94786598b91a24a4b29fe67)
Merge remote-tracking branch 'remotes/origin/branch-2' into branch-2