[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.

1 files changed, 2 insertions, 0 deletions

diff --git a/libmultipath/blacklist.c b/libmultipath/blacklist.c
index 1bb7277..92ab876 100644
--- a/libmultipath/blacklist.c
+++ b/libmultipath/blacklist.c
@@ -3,6 +3,8 @@
  */
 #include <stdio.h>
 
+#include <checkers.h>
+
 #include "memory.h"
 #include "vector.h"
 #include "util.h"