logind: use open_terminal() instead of open()

logind: use open_terminal() instead of open()

The open_terminal() function adds retries in case a terminal
is in the process of being closed when we open it, and should
generally be used to open a terminal. We especially need it
for code that a subsequent commit adds that reopens the terminal
at session shut-down time; such races would be more likely in
that case.

Found by Ray Strode.