changed default nsh serial to UART0 (openSDA usb)

Move prototypes of mkfifo2() from sys/stat.h and pipe2() from unistd.h.  They are non-standard and should not be in such a public place.  They are not in include/nuttx/fs/fs.h (which isn't a great place either).

Add mkfifo2() and pipe2() which are just like mkfifo() and pipe(), but allow control of the size of the underlying, in-memory circular buffer