This adapts the existing in-process logcatcher from tstest/integration
into a public type and uses it on the side of testcontrol. This also
fixes a bug in the Alpine Linux OpenRC unit that makes every value in
`/etc/default/tailscaled` exported into tailscaled's environment, a-la
systemd [Service].EnviromentFile.
Signed-off-by: Christine Dodrill <xe@tailscale.com>