setuserenv: a missing /etc/login.conf file is not an error.

setuserenv: a missing /etc/login.conf file is not an error.
The code to handle LOGIN_SETENV (and thus LOGIN_SETALL) returned
an error if /etc/login.conf could not be opened.  We should simply
return success from setuserenv() in this case like we do for the
other flags.  From Matthew Martin, OK deraadt@