enable maria in the default plugin configuration

config/ac-macros/plugins.m4:
  typo in comment fixed.
  change default plugin configuration to be "default" not "none".
  don't print default plugin configuration in the help text (as it's "default" anyway)
configure.in:
  use "default" plugin configuration by default, not "none"
storage/maria/plug.in:
  enable maria in the default plugin configuration