remove --multiple/-m option from all CLI commands: closes #1262

remove --multiple/-m option from all CLI commands: closes #1262

I personally used "tahoe start/restart -m ../MY-TESTNET/node*" all the time,
to spin up or update a local testgrid while iterating over new code. However,
with the recent switch from "subprocess.Popen(/bin/twistd)" to "import and
call twistd.run()" in scripts/startstop_node.py (yay fewer processes!),
"start -m" broke, and fixing it requires os.fork, which is unavailable on
windows (boo windows!). And I was probably the only one using -m. So in the
interests of uniformity among platforms and simpler code (yay negative code
days!), we're just removing -m from everything. I will start using a little
shell script or something to simulate the removed functionality.

This patch also cleans up CLI-function calling a bit: get the basedir from
the config dict (instead of sometimes from a separate argument), and always
return a numeric exit code.