[Greasemonkey] minor 0.6.3 nit, enabled = disabled

[Michael Kraft]

I figured out the issue with this.

The extension uses a checkbox for the "enable" menu entry.  The checkbox
defaults to unchecked every time the browser is started.  GreaseMonkey doesn't
use whether or not this is checked to determine if GM is enabled or not, it
uses a preference.  The preference and the checkbox can get out of sync.

GreaseMonkey should make sure the checkbox is checked at startup if
GreaseMonkey is enabled.