[checkers] API rewrite

[checkers] API rewrite

The context handling and the layering were poor, so rewrite.

The users now need to :

- alloc a "struct checker"
- select a checker by name : checker_lookup()
- init own checker instance : checker_get()
- set the path file descriptor to check : checker_set_fd()
- use : checker_check()
- release : checker_put()

Checkers now are asked to provide 3 functions :

- foo_init : alloc and initialize the checker context
- foo_free : free the context
- foo      : the checking function proper

These 3 are registered in a static checkers array, along with the checker
name. Lookups are based on checker name.

The users are all updated :

- checker_get is folded into path (re)discovery
- checker_put is folded into path free/orphan

Additional gains :

- directio is updated to use a context to avoid computations upon each
  check
- checkers code is leaner

Please test and report.