In the course of debugging the command-line argument refactoring in
previous commits, I found I wasn't quite sure whether PSCP thought I'd
given it too many arguments, or too few, because it didn't print an
error message saying which: it just printed its giant usage message.
Over the last few years I've come to the belief that this is Just
Wrong anyway. Printing the whole of a giant help message should only
be done when the user asked for it: otherwise, print a short and
to-the-point error, and maybe _suggest_ how to get help, but scrolling
everything else off the user's screen is not a good response to a
typo. I wrote this thought up more fully last year:
https://www.chiark.greenend.org.uk/~sgtatham/quasiblog/stop-helping/
So, time to practise what I preach! The PuTTY tools now follow the
'Stop helping!' principle. You can get full help by saying --help.
Also, when we do print the help, we now exit(0) rather than exit(1),
because there's no reason to report failure: we successfully did what
the user asked us for.
ecfa6b2734