newfs: enable newfs to run on plain files without a bsdlabel

[rew]

Summary

For awhile now it's been possible to newfs a plain file...however, newfs
errors out when the file doesn't have a bsdlabel - this commit changes
that.

[mckusick]

This seems to be a reasonable change.

A side note. the getdisklabel() function is declared static at the top, but static is missing from its definition later in the file. Not sure why the compiler does not flag the inconsistency.