Change default duplex setting and simplify the setup of srrctl.BSIZEPKT.

Change default duplex setting and simplify the setup of srrctl.BSIZEPKT.

During media selection through ifconfig one might not specify the duplex
setting through the mediaopt flag.  In that case the igc(4) would default
to full-duplex.  The problem with this approach is that em(4) defaults to
half-duplex.  Because of that if one connects both NICs and sets media to
e.g. 100baseTX on both of them no carrier will be found.
Fix that by matching igc(4) behaviour with what em(4) does.

Adapted from FreeBSD commits 3b8d04f845b416d29a258658b8a48d1afb4a2e81 and
0eb8cd1de26849cbaefaf3bc59aa9612fa0dfa52 respectively.

ok mbuhl@