shared/logs-show: strip trailing carriage returns at EOL/EOF

shared/logs-show: strip trailing carriage returns at EOL/EOF

When showing logs from a container, we would fail to show various lines:
Oct 29 09:50:51 krowka systemd-nspawn[61376]: Detected architecture x86-64.
Oct 29 09:50:51 krowka systemd-nspawn[61376]: [1B blob data]
Oct 29 09:50:51 krowka systemd-nspawn[61376]: Welcome to Fedora 32 (Rawhide)!
Oct 29 09:50:51 krowka systemd-nspawn[61376]: [1B blob data]

Those are only harmless \r characters that trail the line. We already replace
tabs and strip various ansi characters that we deem inconsequential, so let's
also strip trailing carriage returns. Non-trailing ones are different, because
they change what would be displayed.