Fix exit status. In rev. 1.48 return logic of rtmsg() was confused. This made arp(8) command exit status reversed for -s and -S.
Fix exit status. In rev. 1.48 return logic of rtmsg() was confused. This
made arp(8) command exit status reversed for -s and -S.

Reported by:	sem
MFC after:	2 weeks

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