contrib: add line offset information to file check function 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. This information will be useful to show correct line number in an actual file for errors detected in code fragments embedded in *.t test scripts.

contrib: change return value of file checking function 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. After this patch, caller of _checkfiledata() can count number of errors in each code fragments of an actual file. This will be useful to share --per-file limitation by all embedded code fragments.

contrib: factor out actual error check for file data 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. Newly added _checkfiledata() will be useful to apply checks on code fragments embedded in *.t test scripts.