wutsocket fixes: select() handles timeout wrong, and poll() lacks a safety check (#428)

* - Fix select() to also update the fd_set arguments on timeout condition.
- Move the nfds argument restriction to the nsysnet side.
- Allow user-defined FD_SETSIZE, since newlib's fd_set allow custom sizes.
- Define __socklen_t_defined when socklen_t is typedefed.

* Added safety check for poll(): the nsysnet fd must fit in nsysnet_fd_set.

---------

Co-authored-by: Daniel K. O. (dkosmari) <none@none>

include/sys/select.h

@@ -1,5 +1,7 @@
 #pragma once
 
+#ifndef FD_SETSIZE
 #define FD_SETSIZE 32
+#endif
 
 #include_next <sys/select.h>

include/sys/socket.h

@@ -53,7 +53,11 @@
 #define SO_NOSLOWSTART 0x4000  // disable slowstart
 #define SO_RUSRBUF     0x10000 // enable somemopt provided memory for receive buffer
 
-typedef uint32_t socklen_t;
+#ifndef __socklen_t_defined
+typedef __socklen_t socklen_t;
+#define __socklen_t_defined
+#endif
+
 typedef uint16_t sa_family_t;
 
 struct sockaddr