serial.c:  Fix a race condition noted by Stefan Kolb.  Between the test if the TX buffer is full and entering a critical section, bytes may be removed from the TX buffer making the wait unnecessary.  The unnecessary wait is an inefficiency, but not really a problem.  But with USB CDC/ACM it can be a problem because the entire TX buffer may be emptied when we lose the race.  If that happens that uart_putxmitchar() can hang waiting for data to be removed from an empty TX buffer.

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).