Add pre-exec hook to spawnProcess

New submission from spiv <[email protected]>:

Sometimes it's useful to be able to run some code after fork but before exec.  For example I just had a case where it would be handy to be able to set the BZR_LOG environment variable to `'bzr.log.%d' % os.getpid()` as a cheap solution to something.  mwhudson says he could have used this feature recently as well.

The `subprocess` module provides a `preexec_fn` argument, which provides this.  It also provides a way to work around bugs like http://bugs.python.org/issue1652!  So I think if we provided this feature it would turn out to be useful.

I'm not sure what to provide for platforms without fork.  `preexec_fn` is documented as "Unix only", so that's probably a reasonable answer for Twisted's equivalent.  It's certainly all I care about :)

This ticket is perhaps related to #2415.

----------
Type     : enhancement
Component: core
Keywords : 
Priority : normal
Nosy     : 
----------
http://twistedmatrix.com/trac/ticket/4159