coccicheck: add support for DEBUG_FILE

Enable to capture stderr via a DEBUG_FILE variable passed to
coccicheck. You can now do:

$ rm -f cocci.err
$ export COCCI=scripts/coccinelle/free/kfree.cocci
$ make coccicheck MODE=report DEBUG_FILE=cocci.err
...
$ cat cocci.err

This will be come more useful once we add support to
use more things which would go into stderr, such as
profiling. That will be done separately in another
commit.

Expand Documentation/coccinelle.txt with details.

Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
Acked-by: Nicolas Palix <nicolas.palix@imag.fr>
Signed-off-by: Michal Marek <mmarek@suse.com>