jme: Always save TX interrupt CPUID

This will be used to unify the ifnet.if_cpuid and ifnet.if_npoll_cpuid
igb: Save TX interrupt CPUID into tx_ring struct

This will be used to unify the ifnet.if_cpuid and ifnet.if_npoll_cpuid
kernel/dm: Remove some unused variables.

Also start using a previously unused variable in dm_target_crypt.
kernel/disk: Remove some unused variables and add __debugvar.
kernel: Remove some unused variables in kern/, opencrypto/ and platform/.
kernel/netinet6: Remove some unused variables.