5.19 show-shell option

This is the "show shell commands" option.

This option has some usage constraints. It:

• must be compiled in by defining DEBUG during the compilation.
• may not be preset with environment variables or in initialization (rc) files.

This will cause set -x to be executed in the shell, with the resultant output printed to /dev/tty. This option will have no effect if `--disable-shell' was specified at configure time.