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

A few includes were missing in each file. A definition of
struct polled_mask was moved to fd-t.h. The MAX_POLLERS macro was
moved to defaults.h

Stdio used to be silently inherited from whatever path but it's needed
for list_pollers() which takes a FILE* and which can thus not be
forward-declared.
diff --git a/src/proto_udp.c b/src/proto_udp.c
index 810c314..a1d9176 100644
--- a/src/proto_udp.c
+++ b/src/proto_udp.c
@@ -11,10 +11,10 @@
  */
 
 #include <types/global.h>
-#include <types/fd.h>
+#include <haproxy/fd-t.h>
 #include <types/proto_udp.h>
 
-#include <proto/fd.h>
+#include <haproxy/fd.h>
 
 /* datagram handler callback */
 void dgram_fd_handler(int fd)