check-code: ban grep's context flags (-A/-B/-C) since they're not on Solaris

contrib/check-code.py

@@ -145,6 +145,7 testpats = [
     (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'cp.* -r ', "don't use 'cp -r', use 'cp -R'"),
+    (r'grep.* -[ABC] ', "don't use grep's context flags"),
   ],
   # warnings
   [