_gsu_print_available_commands(): Don't abort if set -e is set.

_gsu_print_available_commands(): Don't abort if set -e is set.

It was pointed out by Congmao Wang that the example hello.bash script
prints only the first subcommand and exits if it is called with an
invalid subcommand and set -e is set. This is due to the "let count++"
statement which returns 1 in the first iteration of the loop since
count is zero.

This patch changes the statement to "let ++count" which returns zero
in all cases.