cut: use error exit-code if a file argument does not open

[mknos]

• When testing cut today I discovered that it uses the idiomatic while(<>) loop for reading input in handle_b() and handle_f()
• If file arguments are provided, all should process correctly in order for a success exit-code
• Failure to open a file argument means process-next-file, not terminate-program
• Hijack the WARN signal to set exit code; in future handle_b() and handle_f() could be restructured if necessary
• If a directory is passed as an argument it is ignored; this is consistent with OpenIndiana & OpenBSD, but GNU cut raises a warning for it
• For now, no attempt is made to suppress line numbers in the warning message

Test from "Git-Bash"...

$ perl -v | perl grep -F built
This is perl 5, version 42, subversion 2 (v5.42.2) built for x86_64-cygwin-thread-multi
$ echo 'y,a,y' > cheer
$ perl cut.pl -c1,2  nothing .. cheer
cut.pl: Can't open nothing: No such file or directory at cut.pl line 122.
y,
$ echo $?
1

[briandfoy]

changes: ensure the an error condition is set and reported in the exit code