Loading Google Analytics as part of an IFRAME that implements an
internal `about:` URL, without being explicitly opt-in is the total
antithesis of privacy respecting.

I should have done this to begin with, and part of me wants to deprecate
all of these options entirely, but people will complain.

Only enable the workaround for the alpha series, and only if the version
is 7.5a2 or older, on the assumption that since it's fixed upstream it
will be reflected in the next Tor Browser alpha build.

This actually is a SelfRando bug.

Work around mass stupidity observed on Ubuntu.

Wasn't so bad to figure out, though upstream still should fall back
gracefully, since the only reason why it crashes horribly is because
pthread_attr_getstack() returns a stack size of 0 for the default thread
when /proc is missing.

In our case we can safely fall back to querying RLIMIT_STACK, because
the sandbox containerization explicity sets it to something large, but
relatively sane.