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

Adds the simulated QSPI (N25Q) flash to the simulation and modify sim up_spiflash.c to enable it to run with different MTD drivers based on config options (currently m25p, sst26 and w25).

PTY: Fix termios read input processing.. what was I thinking?  Also, if some data was read, pty_read() should not block on the empty FIFO.