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;

1. Add poll/select APIs;
2. Add wait queue implementation;
3. Remove DFS_STATUS_* code and use errno;
4. Add pipe API;
5. Use a standalone fops in DFS;

static void copydir(const char * src, const char * dst)

Clang complains about "Null pointer argument in call to string
comparison function". So assert on it.

fix #211

empty is confused in dfs_filesystem_lookup.

clean code with pointer rather than index

decrease indentation to make cleaner code
fix mkfs bug when there is no mkfs implementation

elm fatfs support mkfs with no need to  run dfs_mount elm first; and can mount/umount/mkfs without reset any more

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@2547 bbd45198-f89e-11dd-88c7-29a3b14d5316

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@2546 bbd45198-f89e-11dd-88c7-29a3b14d5316