[Toybox] Notes about netcat toy

[orc]

Hm, seems to be that toybox netcat does not support udp mode and
ipv6. I'm using ipv6 for internal purposes so I see that if I want
ipv6-aware netcat in toybox I need to follow one of two ways here:

- Rewrite current netcat.c logic that it will support ipv6
- Port openbsd's netcat.c into toybox (much work needed plus stripping
  unneeded code, 3-clause BSD)

(Another way is to use openbsd netcat.c directly)

I have dirty (with #if 0 parts) code that supports ipv6 in current
toybox netcat via getaddrinfo, but with udp mode I'm stuck.
Implementing udp option will be useful too.
Rob, do you have some plans about this toy?

Then, some other notes:

- busybox and openbsd netcats doesn't require to specify source
address and port for listen mode. Maybe rewrite netcat that it will
accept something like 'netcat -L 127.0.0.1 8033'?
- Toybox netcat says that it executes program from -e option, but
  actually executes it via toys.optargs.
- New network toys should be ready for ipv6 by default.