setup: more general approach to tty paths

setup: more general approach to tty paths

Unprivileged containers always use an empty lxc.devttydir
option (iow. don't use the /dev/lxc/ subdirectory).
Alpine and Gentoo don't support it in general.

Define a devttydir() sub in Setup::Base which by default
returns "lxc/" or an empty string depending on whether it is
an unprivileged container. Gentoo and Alpine override it
with one which always returns an empty string.