ui/ncurses: Use tty name in default log filename

When we have multiple ncurses UIs running, we'd like to log to separate
files. Currenly, all UIs log to the same file, which makes it diffifult
to determine which UI is logging each message.

This change uses the output of ttyname() (sanitised appropriately) as a
component of the default log filename.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>