Remove the --source-install option and instead make use of --srcdir

to install system tables directly from the source tree (useful for
testing purposes).  This helps clean the script up a lot and clarify
the three possible ways the script can be called (using compiled-in
paths, passing --basedir pointing to alternative install location,
or --srcdir).  Include further tidying as well.
  
This fixes bug#30759.


scripts/mysql_install_db.sh:
  Re-order the logic in printing messages at the end of the
  script, to make it clearer which paths are taken depending
  on tests or outcome of command status.  Tidy some output.