abort cleanly if sandbox directories exist already
reimplement sanity check for space

- mysql-sandbox checks for at least 1G of space before
  proceeding as a basic sanity check.  Most MySQL distributions
  will required a few hundred MB, as will the base mysql footprint
  depending on innodb-log-file-size configuration
unpack symlinks from tarball distributions

- this removes a mistaken hack put in place before to manually
  symlink ha_innodb_plugin.so in 5.1 distributions and otherwise
  restores various symlinks in ./lib
refresh mysql-sandbox usage/example
simplify debugging options

- --log-level removed in favor of simple --quiet (no logging)
  or --debug (all logging enabled)