ipfw(8): Fix most warnings with the default WARNS level.

• Add missing const and static qualifiers.
• Avoid shadowing the global "co" by renaming it to "g_co".
• Avoid mixing signedness in loop bound checks.
• Leave -Wcast-align warnings disabled for now.

Reviewed by: ae, melifaro
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D25456