[libc] Add RT_USING_POSIX macro.

1. Add macro check in rtdebug.h;
2. Use RT_USING_POSIX for poll/select, stdin etc.
3. Split dfs_posix.h to dfs_posix.h, dfs_poll.h and dfs_select.h;

components/dfs/src/poll.c

@@ -23,11 +23,14 @@
  */
 #include <stdint.h>
 
+#include <rthw.h>
 #include <rtdevice.h>
+#include <rtthread.h>
 
 #include <dfs.h>
 #include <dfs_file.h>
 #include <dfs_posix.h>
+#include <dfs_poll.h>
 
 struct rt_poll_node;
 

components/dfs/src/select.c

@@ -25,6 +25,9 @@
 #include <dfs_fs.h>
 #include <dfs_posix.h>
 
+#include <dfs_poll.h>
+#include <dfs_select.h>
+
 int select(int nfds, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout)
 {
     int fd;