- Initialize "rval", which would be used uninitialized
if al or pl options were set.
- Don't pass an empty string to login(1) as a user name
(this could be triggered by entering a name and then killing it
with backspace or ^U.)
- Don't loop endlessly if the al option specifies a bogus (i.e.,
not alphanumeric) auto-login name.
- Don't pass a bogus user name to login(1) if a good name were
entered and then killed with ^U.
- Exit with status 0, not 1, on receiving an EOF character,
since it's not a error condition.
MFC after: 1 week
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