Coccinelle: Add misc/doubleinit.cocci

Find duplicate field initializations.  This has a high rate of false
positives due to #ifdefs, which Coccinelle is not aware of in a structure
initialization.

Signed-off-by: Nicolas Palix <npalix@diku.dk>
Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: Michal Marek <mmarek@suse.cz>