RFR: JDK-8286869: unify os::dir_is_empty across posix platforms [v2]

[Matthias Baesken]

> The os::dir_is_empty coding is very similar across Posix platforms and could be unified to os_posix.cpp .

Matthias Baesken has updated the pull request incrementally with one additional commit since the last revision:

  use readdir, opendir, closedir from global scope

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8750/files
  - new: https://git.openjdk.java.net/jdk/pull/8750/files/ba634945..2229b51d

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8750&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8750&range=00-01

  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8750.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8750/head:pull/8750

PR: https://git.openjdk.java.net/jdk/pull/8750