unix/main: Add #if guard around -v option usage and document -i/-m opts.
unix/main: Add #if guard around -v option usage and document -i/-m opts.

This commit modifies the usage() function to only print the -v option help
text when MICROPY_DEBUG_PRINTERS is enabled.  The -v option requires this
build option to be enabled for it to have any effect.

The usage text is also modified to show the -i and -m options, and also
show that running a command, module or file are mutually exclusive.

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