contrib: refactor preparation logic for patterns of check-code.py

This is a part of preparation to apply checking with check-code.py on
code fragments embedded in *.t test scripts.

Before this patch, preparation logic in _preparepats() of
check-code.py is not reusable. It can handle only module global list
"checks".

This patch splits preparation logic into small internal functions, and
add the loop to invoke them, in order to increase reusability of the
logic.

"c[-2]" is equivalent to "c[3]" for "checks". This patch uses the
former, because it will be more reusable for subsequent patch than the
latter.

contrib/check-code.py

@@ -561,8 +561,7 b' checks = ['
 ]
 
 def _preparepats():
-    for c in checks:
-        failandwarn = c[-1]
+    def preparefailandwarn(failandwarn):
         for pats in failandwarn:
             for i, pseq in enumerate(pats):
                 # fix-up regexes for multi-line searches
@@ -576,10 +575,19 b' def _preparepats():'
                 p = re.sub(r'(?<!\\)\[\^', r'[^\\n', p)
 
                 pats[i] = (re.compile(p, re.MULTILINE),) + pseq[1:]
-        filters = c[3]
+
+    def preparefilters(filters):
         for i, flt in enumerate(filters):
             filters[i] = re.compile(flt[0]), flt[1]
 
+    for cs in (checks,):
+        for c in cs:
+            failandwarn = c[-1]
+            preparefailandwarn(failandwarn)
+
+            filters = c[-2]
+            preparefilters(filters)
+
 class norepeatlogger(object):
     def __init__(self):
         self._lastseen = None