net: sockets: socketpair: Rename read/write signals
net: sockets: socketpair: Rename read/write signals

Rename `write_signal` to `readable` and `read_signal` to `writeable`
which are more meaningful to the actual states they represent, and make
the code analysis easier.

Signed-off-by: Robert Lubos <9d28997c6894a2d0321a7bb399169af7ce99b64e@nordicsemi.no>

galak/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr