grsec compatibility: drop privileges after opening /proc files.

Some /proc files are no longer visible once we drop privileges.
So try to open them first, and only drop privileges before entering
the main loop.
Drop privileges in dfstat: no need to stay root.
Drop privileges in procstats after opening /proc files.
Drop privileges in ifstats after opening /proc file.

Will have to find a way to remove all this code duplication later.
Drop privileges in iostats after opening /proc file.

More code duplication :(
Add collector for stats published by OpenTSDB over HTTP.

Signed-off-by: Benoit Sigoure <tsunanet@gmail.com>
opentsdb.sh: exit 13 if there is no TSD to connect to.