#18678: Correct names of spwd struct members.

The old names (sp_nam and sp_pwd) are kept for backward compatibility.  Since
this is a long standing bug that hasn't caused any real-world problems, I'm
not backporting it.  However, it is worth fixing because the corrected names
match the documentation, and more importantly now match the C struct, just
like the other struct members.

Patch by Vajrasky Kok.