Bug fix: incomplete registering of cloned checkers.

Scenario:

- a C file is recognized as having filetype cpp
- run :SyntasticInfo
- run :setf c
- run :SyntasticInfo again

The only checker recognized will be oclint.  The reason is, oclint is
registered as a c checker when called from cpp initialization, and that
prevents registering other c checkers when calling ':setf c'.

Solution: always initialize all checkers for a filetype.