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/dns.c b/src/dns.c
index 0390212..a74bd36 100644
--- a/src/dns.c
+++ b/src/dns.c
@@ -38,7 +38,7 @@
 #include <proto/cli.h>
 #include <proto/checks.h>
 #include <proto/dns.h>
-#include <proto/fd.h>
+#include <haproxy/fd.h>
 #include <proto/http_ana.h>
 #include <proto/http_rules.h>
 #include <proto/log.h>