login: Restore tty size after calling vhangup()

login: Restore tty size after calling vhangup()

If login receives the tty to work on via stdin, stdout and stderr,
login might end up closing the remaining open file descriptors to
the tty just before it calls vhangup(). When the last open file
descriptors to a tty are closed, it's configured size is reset to
0x0. To avoid this from happening, save the size before closing
the stdin, stdout and stderr file descriptors and reapply the size
after the tty is re-opened.

Fixes #1484