Add optional poll() implementation under MG_ENABLE_POLL

This adds the ability to substitute select() with poll() on supported
unix environments when MG_ENABLE_POLL is defined. A make flag called
USE_POLL is provided as well.
Using poll() removes the limitation of FD_SETSIZE concurrent sockets,
generally 1024 on Linux environments.

mongoose.h

@@ -436,7 +436,11 @@ extern int SockSet(SOCKET hSock, int Type, int Prop, void *pbuf, int size);
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#if defined(MG_ENABLE_POLL) && MG_ENABLE_POLL
+#include <poll.h>
+#else
 #include <sys/select.h>
+#endif
 #include <sys/socket.h>
 #include <sys/stat.h>
 #include <sys/time.h>
@@ -578,6 +582,10 @@ int sscanf(const char *, const char *, ...);
 #define MG_ENABLE_MIP 0
 #endif
 
+#ifndef MG_ENABLE_POLL
+#define MG_ENABLE_POLL 0
+#endif
+
 #ifndef MG_ENABLE_FATFS
 #define MG_ENABLE_FATFS 0
 #endif