add ether_aton[_r] and ether_ntoa[_r] functions

based on patch by Strake with minor stylistic changes, and combined
into a single file. this patch remained open for a long time due to
some question as to whether ether_aton would be better implemented in
terms of sscanf, and it's time something was committed, so here it is.

include/netinet/ether.h

@@ -0,0 +1,14 @@
+#ifndef _NETINET_ETHER_H
+#define _NETINET_ETHER_H
+
+#include <netinet/if_ether.h>
+
+char *ether_ntoa (const struct ether_addr *);
+
+struct ether_addr *ether_aton (const char *);
+
+char *ether_ntoa_r (const struct ether_addr *, char *);
+
+struct ether_addr *ether_aton_r (const char *, struct ether_addr *);
+
+#endif