Staticize several variables that are used exclusively in bsnmpd/main.c and
bsnmpd/trap.c

This fixes several -Wmissing-variable-declaration warnings noted by clang

Differential Revision: https://reviews.freebsd.org/D4718
MFC after: 1 week
Reported by: Jenkins
Reviewed by: araujo
Sponsored by: EMC / Isilon Storage Division