use a new macro NELEM() to count the number of element in a static sized array.
Added a Coccinelle test for NELEM.

  Introduce a new directory in tests for .cocci files.

  "Coccinelle is a program matching and transformation engine which
  provides the language SmPL (Semantic Patch Language) for specifying
  desired matches and transformations in C code."
  - see http://coccinelle.lip6.fr/
Use NELEM() where appropriate.
Added cocci scripts

  A README is provided in tests/cocci with a very short usage intro.
  Most of the scripts in tests/cocci where taken from the examples found
  in the Coccinelle website. Scripts in tests/cocci/pkg were adapted or
  writting for pkg.
badzero.cocci fixes.
pkg_jobs.c: Removed a useless test detected by notnull.cocci.
Typos in the unchecked_malloc cocci file.
Document the plist format

Fixes: #627