add MTD support for Micron N25Qxxx family of QSPI flash

fs/inode/, fs/vfs/, and sched/task/:  File and socket descriptors are no longer allocated for kernel threads.  They must use SYSLOG for output and the low-level psock interfaces for network I/O.  This saves a little memory which might be important for small footprint configurations.

Fix the STM32L4 SPI driver.  That SPI driver is quite different. They now handle frames of arbitrary size between 4 and 16 bits. It was broken before a new bit has to be set (rx fifo threshold) to handle <= 8-bit transactions. If not set, the default is 16-bit packed >=8-bit frames and the RXNE bit is never set (it is set when 16-bits are received). weird things as always.

This also add 8-bit access routines to the data register, because a 16-bit access to the data register when the frame size is below 9 bits is interpreted as a packed dual frame exchange.

configs/nucleus2g:  Remove this board from the configs/ directory.  A snapshot is still available in the Obsoleted repository

Fix an error when a task with open message queue descriptors is killed via task_delete().  Noted by Anton Gropyanov.

Signals:  Correct implementation of sigset().  It is not just signal() with a different name; it has some additional signal mask handling functionality

libc/signal and include/signal.h: Rename sigset() to signal().  sigset() is the System V function; signal() is the obsoleted POSIX name.  These seem to be equivalent.  Neither are relevant in modern POSIX programming but the name signal() probably has better credentials.  sigset() is now relegated to a #define in signal.h that makes it equivalent to signal().