Use -ltinfo instead of -lncurses.

In tools except guestfish, we don't use ncurses, or even curses, just
2 termcap functions and a global variable.  These are provided by
libtinfo, so just link to that.

In guestfish we use readline which needs ncurses.  Leave that alone.
v2v: Ignore small filesystems when checking for sufficient free space.
v2v: Document minimum free filesystem space requirements.