tools: bpftool: show that `opcodes` or `file FILE` should be exclusive

For the `bpftool prog dump { jited | xlated } ...` command, adding
`opcodes` keyword (to request opcodes to be printed) will have no effect
if `file FILE` (to write binary output to FILE) is provided.

The manual page and the help message to be displayed in the terminal
should reflect that, and indicate that these options should be mutually
exclusive.

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