[hci] Rename "item" command's first parameter from "label" to "name"

[hci] Rename "item" command's first parameter from "label" to "name"

Switch terminology for the "item" command from "item <label> <text>"
to "item <name> <text>", in preparation for repurposing the "item"
command to cover interactive forms as well as menus.

Since this renaming affects only a positional parameter, it does not
break compatibility with any existing scripts.

Signed-off-by: Michael Brown <mcb30@ipxe.org>