Support setting the rotateLength parameter to None from twistd

[robertc]

twistd -l foo.log sets a log file called foo.log which external rotation can be done on by 'mv foo.log rotated.log; kill -USR1 $(< twisted.pid)'.

This is great but it leaves autorotation on, which makes this a bit messy.

It would be really nice if there was a way to completely disable log rotation from the twistd command line. E.g.

twistd -l foo.log --rotate-at=None

This would pass through to the LogFile.fromFullPath() function, and need to translate 'None' to None.

Sadly I don't know the twistd command line glue stuff, because this seems conceptually trivial, and it would be really really useful :)

[exarkun]

Duplicate of #3363