netmap: Fix compiler warnings in tools

[markj]

Summary

• Remove write-only variables, or hide them in cases where their use is conditional or commented out.
• Check for errors from cmd_apply() in nmreplay.
• Use ANSI C definitions.

[markj]

tools/tools/netmap/lb.c

392

This one gives a compile error with clang 15.

[vmaffione]

tools/tools/netmap/pkt-gen.c

1524

why not dropping this?

1550–1551

why not dropping this? It looks like leftover code

[markj]

tools/tools/netmap/pkt-gen.c

1550–1551

There are some commented-out prints that reference rx. Should I remove those as well?

[markj]

Drop commented-out lines in pkt-gen.

[vmaffione]

Yeah, if not used since you are here please clean it up.