Bug 1494436 - Unset MOZ_APP_LAUNCHER for external MIME handlers

Bug 1494436 - Unset MOZ_APP_LAUNCHER for external MIME handlers

If Thunderbird sets this in its startup script (as it does in Debian
and Fedora), Firefox does not set this in its startup script (it doesn't
in Debian), and Firefox is the handler for clicking a link in
Thunderbird, then Firefox will think it is part of Thunderbird and offer
to make Thunderbird (not Firefox!) the default browser. If the user
accepts this, it will break the ability to open normal HTTP links and
HTML files from other applications.

The same would be true for any other pair of Mozilla-based applications.
To avoid this, unset MOZ_APP_LAUNCHER for the Firefox child process.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1494436
Bug-Debian: https://bugs.debian.org/948691

Gbp-Pq: Topic fixes
Gbp-Pq: Name Bug-1494436-Unset-MOZ_APP_LAUNCHER-for-external-MIME-hand.patch