libifconfig: Fix SFP table generators for Lua 5.5

[rmoeller]

Summary

This fixes Lua 5.5 compat in the .c template, where the first loop
variable '_' becomes const in Lua 5.5. Use select() to discard inside
the loop instead.

While here, fix a mismatched variable name in the corresponding header
template.

Reported by: Austin Horstman <[e-mail]>
Closes: https://github.com/freebsd/freebsd-src/pull/2473

[kevans]

lib/libifconfig/libifconfig_sfp_tables.tpl.c

63

TIL about select(), neat.