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.

This change provides an option to add /dev/urandom to all architectures.  The pseudo-random algorithm I choose strikes an arguably-good balance between being "random" and small/fast enough for 8/16 bit MCUs.  It’s the well-documented xorshift128 algorithm.  It has an internal state of 128 bits that can be [re-]seeded with a write.

PTY: Add some encode to reduce the likely of misinterpreting the return value of the file open() method

EXEEXT only needs to be appended once

PTY: Add ioctls to support locking and unlocking of the slave.  LIBC: Add implementation of unlockpt()