nlmsg: Improve payload printing
nlmsg: Improve payload printing

It makes more sense to use isprint() than isalnum() because we use non
alphanumeric characters like '%', '_', etc. And, in case of non
printable character, print a space is preferable to print a NULL (0) in
order to keep alignment.

Before:
...
|00012|--|00002|	|len |flags| type|
| 5f 5f 73 65  |	|      data      |	   s e
| 74 25 64 00  |	|      data      |	 t  d
...

After:
...
|00012|--|00002|	|len |flags| type|
| 5f 5f 73 65  |	|      data      |	 _ _ s e
| 74 25 64 00  |	|      data      |	 t % d
...

Signed-off-by: Carlos Falgueras García <f404c3030e3051e6f14dcb23c3c0aa70841a13b6@riseup.net>
Signed-off-by: Pablo Neira Ayuso <707d14912bb250caf67dfe0ea4035681fbfc4f56@netfilter.org>

greearb/libmnl-ct,greearb/libmnl-ct