Sanitize environment before re-execing under 'ip netns exec'.

This program was already doing aggressive environment sanitization
before invoking the isolated child, but was not doing the same for
'ip netns exec' (used for ISOL_NETNS= mode) -- and that one happens
as root.  And we don't especially trust a shell not to get involved.
Remove obsolete openvpn-* scripts.
Remove obsolete PlanetLab scripts.
blah
actually this isn't even needed anymore
Additional defensiveness against malicious environment settings.

'isolate' now aggressively sanitizes its own environment before doing
anything whatsoever that could cause invocation of a subprocess, in
particular before the self-reexec prompted by ISOL_NETNS=.  Largely
prompted by Shellshock.
Yet more bugfixes.
Better handling of redirection chains involving <link>.

In particular, if the page 3xx's us to a destination which has a
<link rel="canonical"> pointing back to the original, do not follow
the <link>.  This comes up a lot with geotargeting and can put us in
an infinite loop.
Replication.
Merge laguz:tbbscraper