netmap: Fix compiler warnings in tools

• 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.

Reviewed by: vmaffione
MFC after: 2 weeks
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D38752