ipfilter: Interface name must not extend beyond end of buffer

[cy]

Summary

sifpidx (an interface name) cannot extend beyond the end of the
fr_names buffer.
We do the validation for fr_sifpidx here because it is a union that
contains an offset only when fr_sifpidx points to an interface name,
an offset into fr_names. The union is an offset into fr_names in this
case only.

Note that sifpidx is only used in ipf_sync() which implments ipf -y.

Reported by: Ilja Van Sprundel <[e-mail]>
MFC after: 1 week

[glebius]

sys/netpfil/ipfilter/netinet/fil.c

242

Can also be declared const.

[cy]

Add const to interr_tbl

[cy]

Whitespace change.

[cy]

Made a couple of changes to ubreak the build.