Author: exarkun
Reviewer: glyph, Screwtape, jesstess, therve, ivank
Fixes: #3526

Factor the duplicate `sys.path` manipulation done by the command line scripts
to share a single implementation.


Also change trial and twistd to always add the current working directory to
`sys.path` (previously it would be added unless running as root or on Windows).


git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/trunk@32202 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb