logind: rename vtconsole to seat0

The seat->vtconsole member always points to the default seat seat0. Even
if VTs are disabled, it's used as default seat. Therefore, rename it to
seat0 to correctly state what it is.

This also changes the seat files in /run from IS_VTCONSOLE to IS_SEAT0. It
wasn't used by any code, yet, so this seems fine.

While we are at it, we also remove every "if (s->vtconsole)" as this
pointer is always valid!