tools: bpftool: make error message from getopt_long() JSON-friendly

If `getopt_long()` meets an unknown option, it prints its own error
message to standard error output. While this does not strictly break
JSON output, it is the only case bpftool prints something to standard
error output if JSON output is required. All other errors are printed on
standard output as JSON objects, so that an external program does not
have to parse stderr.

This is changed by setting the global variable `opterr` to 0.
Furthermore, p_err() is used to reproduce the error message in a more
JSON-friendly way, so that users still get to know what the erroneous
option is.

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>