WP_Filesystem_*::mkdir() untrailingslash path consistently, don't waste time...

WP_Filesystem_*::mkdir() untrailingslash path consistently, don't waste time attempting to create an "empty" path. See #15575. Props lordandrei and SergeyBiryukov for initial patches.

git-svn-id: https://core.svn.wordpress.org/trunk@18964 1a063a9b-81f0-0310-95a4-ce76da25c4cd