Safer and more consistent umask handling

Old Behaviour
=============
umask setting only controlled permissions on unix sockets and workertmp files.
And the default value for this setting is 0 (world writable files)
In addition to this enabling gunicorn daemon mode implicitly sets umask to 0
resulting in all other files created by gunicorn (log, pid) and all files
created by a wsgi app becoming world writable, regardless of the umask
setting.

New behaviour
=============
The umask default valueis None (instead of 0) meaning leaving umask
unmodified. If umask is specified it applies to all files created by
gunicorn (unix socket, pid file, log file, workertmp) as well as the wsgi app
itself.

Conflicts:
	gunicorn/config.py
	gunicorn/util.py