fatfs: mkfs时增加FM_SFD参数，减少文件系统对flash空间的占用

注: armclang编译器报错

Signed-off-by: MurphyZhao <d2014zjt@163.com>

[DFS] Update net_sockets.c

修复创建socket失败仍然返回有效的fd

libc_stdio_set_console/newlib uses fopen to open console device, which
has O_CREAT flag and cause fault.

in components/dfs/filesystems/net/netinet/in.h, add IN6_IS_ADDR_MULTICAST support

fix a bug cannot open file

[DFS] fix ROMFS get root directory bug

1. Remove INIT_FS_EXPORT and change INIT_DEVICE_EXPORT as the first item
in the initalization thread.
2. Move the eth_system_device_init into INIT_PREV_EXPORT item.

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;

[DFS] To make sure the "console" device exist or not when initialize
console device.

[DFS] Fix the ramfs issue.

修正components.h中使能RT_USING_DFS_RAMFS宏时，加入ramfs.h头文件

比如实际目录为abcde，msh执行 cd abc也能执行成功，判断名称时存在bug

Signed-off-by: pangwei <271372443@qq.com>

To avoid same file name, therefore change the netdb.c to lwip_netdbc in
dfs_lwip.

System will crash when the romfs is erased. Add checks before using them
to avoid it.

This issue causes fstat & lseek not correct.

I have to mention that this bug is introduced by myself, in the last
commit <fd07f30f511335f506ae6557e300093d4dde421e> to dfs_nfs.c

more infomation:
http://www.rt-thread.org/phpBB3/topic2965.html

_TIME_T_DEFINED was pre-defined in order to build jffs2 with VC.
While it may cause many side effects.

found by haitao5198
clear mount parition device talbe when jffs2 mount falied

found by haitao52198
URL:http://www.rt-thread.org/phpBB3/viewtopic.php?f=3&t=3112&p=17153#p17153