Bug #3 - -S option ignored if SIOCETHTOOL succeeds

This fixes http://sourceforge.net/p/nicstat/bugs/3/ by applying the
patch I provided in that that bug report.

This patch may not be ideal, and should be revisited for completeness,
but does resolve the issue for me.
Bug #4 - Utilization always computed as half duplex

This fixes http://sourceforge.net/p/nicstat/bugs/4/ by applying the
patch I provided in that that bug report.

When utilization is calculated, nicp->duplex checks for the value of
"2" for a full duplex link, but on Linux this value is "1" (see
/usr/include/linux/ethtool.h). Fix is to use DUPLEX_FULL #define.
Bug #5 - Remove unnecessary #ifdef block

This fixes http://sourceforge.net/p/nicstat/bugs/5/ by applying the
patch I provided in that that bug report.

An #ifdef for linux vs solaris is not needed as the DUPLEX_ #defines
can be used consistently instead.