cut: promptly diagnose write errors, continued

cut: promptly diagnose write errors, continued

* src/cut.c: Complete the error-handling improvements started in
commit e0a4a60af5, by adding a couple of remaining checks for putchar().
While there, sprinkle a few rather useful comments, and perform a few
small code cleanups, to make the code and the comments more uniform
and more conformant to the official coding style.  Also make the help
message slightly more uniform.