RFR: Make Host::user return Optional<String>

[Erik Helin]

Hi all,

please review this refactoring that makes `Host::user` return an
`Optional<String>` to support cases when a username does not map to a user.

Testing:
- `make test` passes on Linux x64

Thanks,
Erik

-------------

Commits:
 - 61f86b04: Make Host::user return Optional<String>

Changes: https://git.openjdk.java.net/skara/pull/435/files
 Webrev: https://webrevs.openjdk.java.net/skara/435/webrev.00
  Stats: 60 lines in 11 files changed: 32 ins; 0 del; 28 mod
  Patch: https://git.openjdk.java.net/skara/pull/435.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/435/head:pull/435

PR: https://git.openjdk.java.net/skara/pull/435