Fixed upstream #505 : spurious warning on adm5 terminal

[fstark]

Fix for issue #505 ("Wrong warning when running on Lear Siegler ADM-5 terminal")

The cheaper function that compares two ways of using termcap sets missing_cap to 1 if both ways are unavailable. This function is used to choose between using "al" or "sr" ("add line" or "scroll reverse") to add a line at the top of the screen. None of those are available on an ADM5 terminal.

However, less already has support for terminal that lacks reverse scroll, and is correctly setting the no_back_scroll, making the warning wrong.

Source code already had a provision to avoid using cheaper if none of the capabilities were available, but it was only used for OS2. I enabled this for all builds. I don't see any potential for any regression.