'tahoe start': stop using the contents of .tac files

Instead of constructing a sys.argv for 'twistd' that reads the node's
.tac file, we construct arguments that tell twistd to use a special
in-memory-only plugin that creates the desired node instance directly.

We still use the name of the .tac file to decide which kind of instance
to make (Client, IntroducerNode, KeyGenerator, StatsGatherer), but never
actually read the contents of the .tac file. Later improvements could
change this to look inside the tahoe.cfg for a nodetype= directive, etc.

This also makes it easy to have "tahoe start BASEDIR" pass the rest of
its arguments on to twistd, so e.g. "tahoe start BASEDIR --nodaemon
--profile=prof.out" does what you'd expect "twistd --nodaemon
--profile=prof.out" to do. "tahoe run BASEDIR" is thus simply aliased to
"tahoe start BASEDIR --nodaemon". This removes the need to special-case
--profile and --syslog.

I also removed some of the default logging behavior:

 before:
  'tahoe start' = 'twistd --logfile BASEDIR logs/twistd.log'
  'tahoe start --profile' adds '--profile=profiling_results.prof --savestats'
  'tahoe run' = 'twistd --nodaemon --logfile BASEDIR/logs/tahoesvc.log'

 after:
  'tahoe start' = 'twistd --logfile BASEDIR logs/twistd.log'
     unless --logfile, --nodaemon, or --syslog are passed
  'tahoe start --profile' invalid, use 'tahoe start --profile=OUTPUT'
  'tahoe run' = 'twistd --nodaemon'
     so log messages go to stdout

This finally enables 'tahoe run' to work with all node types, including
the key-generator and stats-gatherer.

It gets 'tahoe start' one step closer to accepting --reactor= . To
actually accomplish this will require this file, the enclosing
__init_.py files, and everything they import to avoid importing the
reactor. (if anything imports twisted.internet.reactor before
startstop_node.start() gets to run, then --reactor= comes too late).
That will take a lot of work, and requires lazy-loading of many core
libraries (foolscap.logging in particular), and removing a lot of code
from src/allmydata/__init__.py .
startstop_node.py: rename internal command and twistd plugin class used to start tahoe nodes.

Signed-off-by: David-Sarah Hopwood <david-sarah@jacaranda.org>
startstop_node.py: improve error reporting and help.
startstop_node.py: improvements to comments.

Signed-off-by: David-Sarah Hopwood <david-sarah@jacaranda.org>