usr.bin/login: send errors to console if syslog isn't running

I was debugging why login(1) wasn't working as expected on a minimal
MFS_ROOT disk image. This image doesn't have syslogd running so the
warnings were lost and I had to use GDB to find out why login(1) was
failing (missing PAM libraries) instead of being able to see it in
the console output.

MFC after: 1 week
Reviewed By: pfg
Differential Revision: https://reviews.freebsd.org/D30892