REORG: include: move checks.h to haproxy/check{,-t}.h

All includes that were not absolutely necessary were removed because
checks.h happens to very often be part of dependency loops. A warning
was added about this in check-t.h. The fields, enums and structs were
a bit tidied because it's particularly tedious to find anything there.
It would make sense to split this in two or more files (at least
extract tcp-checks).

The file was renamed to the singular because it was one of the rare
exceptions to have an "s" appended to its name compared to the struct
name.
diff --git a/include/types/server.h b/include/types/server.h
index 38326b0..1f9ef1f 100644
--- a/include/types/server.h
+++ b/include/types/server.h
@@ -25,6 +25,7 @@
 #include <netinet/in.h>
 #include <arpa/inet.h>
 
+#include <haproxy/check-t.h>
 #include <haproxy/counters-t.h>
 #include <haproxy/dns-t.h>
 #include <haproxy/api-t.h>
@@ -41,7 +42,6 @@
 #include <types/queue.h>
 #include <types/ssl_sock.h>
 #include <haproxy/task-t.h>
-#include <types/checks.h>
 
 
 /* server states. Only SRV_ST_STOPPED indicates a down server. */