Rename --enable-expensive-hardening configure option

It is renamed to --enable-fragile-hardening.

TROVE-2017-001 was triggerable only through the expensive hardening which is
making the tor daemon abort when the issue is detected. Thus, it makes tor
more at risk of remote crashes but safer against RCE or heartbleed bug
category.

Fixes #21290.

Signed-off-by: David Goulet <dgoulet@torproject.org>