Update dsl#local_user method

- uses more portable `Etc#getlogin`.
- previous implementation with `whoami` returns a string with a newline
  at the end. This implementation returns a clean string. As a
  consequence `revisions.log` does not have a trailing `;` character at
  the end.
Merge pull request #1019 from bruno-/tweak_revision_log

Update dsl#local_user method