Improve robustness of owner/mode settings.

[delphij]

Summary

• Run fcgiwrap under target credential and only change ownership if the credential do not match the current socket_owner/group setting.
• Use umask instead of chmod.
• Use nobody/nogroup for default credentials instead of root.

[mat]

Looks ok to me, might want to explain a bit the umask thing, took me a few minutes to see what you were doing.