FAT: Fix some errors that I introduced in my review of Alan's patch.  My screw-up, not Alan's.  Sorry.

romfs:  Simplify fstat() implementation.  It is not necessary to save the file type at open,  We know in this context that the file is a regular file.

rename(): Correct more issues.  (1) Move to the root directory in the pseudo file system, (2) Fix path naming calculation when the path is the root directory of a mounted file system, and (3) dont't do the rename if the source and destination of the rename are the same.

Add logic to VFS rename:  If target of rename exists and is a directory, then the source file should be moved 'under' the target directory.  POSIX also requires that if the target is a file, then that old file must be deleted.

Minor typos

Approved-by: Gregory Nutt

Apparently setvbuf() size can be nonzero with _IONBF.  That makes no sense, but is necessary if setbuf() is to work as it is defined at OpenGroup.org.

Merged in david_s5/nuttx-6/david_s5/typo-up_exitc-edited-online-with-bitbuck-1486672675169 (pull request #207)

Typo up_exit.c edited online with Bitbucket

Approved-by: Gregory Nutt

STM32 & STM32F7 Fixes the bkp reference counter issue

Approved-by: Gregory Nutt