Newlines are ignored when testing if text can fit one screen

[notramo]

slit ignores newlines in files when tests if output can fit one screen.
For example, the terminal is 50 lines high.
If the file has less non-empty lines than the number of terminal lines (e.g. 49), but it has empty lines also (e.g. 3), the ncurses UI is not launched although the total number of lines is more than the terminal height.