check-code: add a rule to forbid "cp -r"

See the commit message of the previous patch for the reason. In short,
according to the current POSIX standard, "-r" is "removed", and "-R" is the
current standard way to do "copy file hierarchies".

contrib/check-code.py

@@ -142,7 +142,8 b' testpats = ['
     (r'\|&', "don't use |&, use 2>&1"),
     (r'\w =  +\w', "only one space after = allowed"),
     (r'\bsed\b.*[^\\]\\n', "don't use 'sed ... \\n', use a \\ and a newline"),
-    (r'env.*-u', "don't use 'env -u VAR', use 'unset VAR'")
+    (r'env.*-u', "don't use 'env -u VAR', use 'unset VAR'"),
+    (r'cp.* -r ', "don't use 'cp -r', use 'cp -R'"),
   ],
   # warnings
   [