REORG: include: split mailers.h into haproxy/mailers{,-t}.h

The file mostly contained struct definitions but there was also a
variable export. Most of the stuff currently lies in checks.h and
should definitely move here!
diff --git a/src/mailers.c b/src/mailers.c
index 4335453..6014894 100644
--- a/src/mailers.c
+++ b/src/mailers.c
@@ -12,6 +12,6 @@
 
 #include <stdlib.h>
 
-#include <types/mailers.h>
+#include <haproxy/mailers-t.h>
 
 struct mailers *mailers = NULL;