RFR: 8274721: UnixSystem fails to provide uid, gid or groups if no username is available

[Weijun Wang]

Two changes:

1. Calculation of groups is put in a separate block.
2. Add fallback for uid and gid

No regression test. Hard to reproduce bud reporter's environment in jtreg.

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Commit messages:
 - 8274721: UnixSystem fails to provide uid, gid or groups if no username is available

Changes: https://git.openjdk.java.net/jdk/pull/5808/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5808&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274721
  Stats: 47 lines in 1 file changed: 15 ins; 6 del; 26 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5808.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5808/head:pull/5808

PR: https://git.openjdk.java.net/jdk/pull/5808