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.

As mentioned above, this change has impact to `revisions.log` file (for the better I think). Here's the `revisions.log` before the change:

    Branch master (at 94e6a4f) deployed as release 20140420124518 by user;
    Branch master (at 94e6a4f) deployed as release 20140420130613 by user;

after:

    Branch master (at 94e6a4f) deployed as release 20140420124518 by user
    Branch master (at 94e6a4f) deployed as release 20140420130613 by user

Notice the semicolon at the end is gone - that's the only change.