subcommand: Overhaul _gsu_print_available_commands().

This function is rather confused. It uses gsu_short_msg() to print
the header to stderr while the list of subcommands is printed to
stdout. This is "fixed" by executing all commands in a subshell and
redirecting stderr to stdout.

It's easier to just print everything to stdout. This patch also
renames the loop variable i to cmd, declares $count as a local
integer variable and switches to single quoted string constants.