Improve sysctl handler code a bit

if_*em.c:
- Whitespace cleanup.

if_igb.c:
- Fix sysctl typing so it compiles with #define notyet in sys/sysctl.h.
- Diff reduce with if_*em.c wherever possible and sensical.
- Whitespace cleanup.

Signed-off-by: Garrett Cooper <yanegomi@gmail.com>