[PATCH] Fix NoScript IPC for about:blank by whitelisting messages

If about:blank is the homepage (or has been passed as a command line
parameter), NoScript will send a message named "fetchChildPolicy"
_before_ "started". Torbutton would then send its "updateSettings" too
soon, resulting in the dreaded error "Could not establish connection.
Receiving end does not exist" (see bug 26520).

Fix this by whitelisting the relevant messages from NoScript: "started"
and also "pageshow" for a slightly more graceful failure mode in case
Torbutton somehow misses NoScript startup.

Trac:
Username: rustybird

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