checksrc: enable strict mode and warnings

Enable strict and warnings mode for checksrc to ensure we aren't missing
anything due to bugs in the checking code. This uncovered a few things
which are all fixed in this commit:

* several variables were used uninitialized
* several variables were not defined in the correct scope
* the whitelist filehandle was read even if the file didn't exist
* the enable_warn() call when a disable counter had expired was passing
  incorrect variables, but since the checkwarn() call is unlikely to hit
  (the counter is only decremented to zero on actual ignores) it didn't
  manifest a problem.

Closes #3090
Reviewed-by: Daniel Stenberg <daniel@haxx.se>
Reviewed-by: Marcel Raad <Marcel.Raad@teamviewer.com>