tools: bpftool: read from stdin when batch file name is "-"

Make bpftool read its command list from standard input when the name if
the input file is a single dash.

Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>