extmod/modlwip: Add support for polling UDP sockets for writability.
extmod/modlwip: Add support for polling UDP sockets for writability.

selste/micropython,pozetroninc/micropython,selste/micropython,henriknelson/micropython,henriknelson/micropython,tobbad/micropython,trezor/micropython,bvernoux/micropython,pramasoul/micropython,adafruit/circuitpython,MrSurly/micropython,pramasoul/micropython,selste/micropython,pfalcon/micropython,tobbad/micropython,kerneltask/micropython,pramasoul/micropython,selste/micropython,pozetroninc/micropython,bvernoux/micropython,pozetroninc/micropython,pozetroninc/micropython,pfalcon/micropython,MrSurly/micropython,bvernoux/micropython,henriknelson/micropython,trezor/micropython,kerneltask/micropython,MrSurly/micropython,pramasoul/micropython,henriknelson/micropython,MrSurly/micropython,trezor/micropython,pramasoul/micropython,selste/micropython,bvernoux/micropython,adafruit/circuitpython,adafruit/circuitpython,pfalcon/micropython,trezor/micropython,adafruit/circuitpython,pozetroninc/micropython,MrSurly/micropython,adafruit/circuitpython,henriknelson/micropython,tobbad/micropython,bvernoux/micropython,kerneltask/micropython,tobbad/micropython,pfalcon/micropython,tobbad/micropython,kerneltask/micropython,trezor/micropython,kerneltask/micropython,pfalcon/micropython,adafruit/circuitpython