[Rev 01] RFR: Add support for adding extra headers to mails sent to mailing lists

[Robin Westberg]

The pull request has been updated with a complete new set of changes (possibly due to a rebase).

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Commits:
 - a38ab5c9: Fix merge conflicts
 - 71ace6ff: Merge remote-tracking branch 'upstream/master' into extra_headers
 - 40851f43: Merge remote-tracking branch 'upstream/master' into extra_headers
 - 7fcd00bc: Add support for adding extra headers to mails sent to mailing lists

Changes: https://git.openjdk.java.net/skara/pull/158/files
 Webrev: https://webrevs.openjdk.java.net/skara/158/webrev.01
  Stats: 44 lines in 5 files changed: 28 ins; 0 del; 16 mod
  Patch: https://git.openjdk.java.net/skara/pull/158.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/158/head:pull/158

PR: https://git.openjdk.java.net/skara/pull/158