engines.js: Disable hyperspace mode on entry to main menu.

  Fix bug where exiting the game back to the main menu
    while the ship is in hyperspace (not hyperwarp, but
    when hyperwarp reaches infinity/100.0 velocity)
    puts the game in a broken state where the engines
    are permanently at 100.0 velocity.
field.js: Make missiles larger and bogeys/the player smaller.

  Should make it easier to block missiles.