Clean up error handling in fileupdate(), which now returns 0 on success

Clean up error handling in fileupdate(), which now returns 0 on success
instead of a boolean. This replicated through he front-end sub-functions
relating to add, delete, modify entries in passwd & group files
Errno is now preserved so output of errc()/warnc() will be less obfuscated
by subsequent errors when reporting the problem.

Add more intelligent error handling when attempting to modify/delete NIS
entries with no corresponding local database entry.

[MFC to stable in a couple of weeks to keep both in sync]