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.

src/network/ether.c

@@ -0,0 +1,43 @@
+#include <stdlib.h>
+#include <netinet/ether.h>
+#include <stdio.h>
+
+struct ether_addr *ether_aton_r (const char *x, struct ether_addr *p_a)
+{
+	struct ether_addr a;
+	char *y;
+	for (int ii = 0; ii < 6; ii++) {
+		unsigned long int n;
+		if (ii != 0) {
+			if (x[0] != ':') return 0; /* bad format */
+			else x++;
+		}
+		n = strtoul (x, &y, 16);
+		x = y;
+		if (n > 0xFF) return 0; /* bad byte */
+		a.ether_addr_octet[ii] = n;
+	}
+	if (x[0] != 0) return 0; /* bad format */
+	*p_a = a;
+	return p_a;
+}
+
+struct ether_addr *ether_aton (const char *x)
+{
+	static struct ether_addr a;
+	return ether_aton_r (x, &a);
+}
+
+char *ether_ntoa_r (const struct ether_addr *p_a, char *x) {
+	char *y;
+	y = x;
+	for (int ii = 0; ii < 6; ii++) {
+		x += sprintf (x, ii == 0 ? "%.2X" : ":%.2X", p_a->ether_addr_octet[ii]);
+	}
+	return y;
+}
+
+char *ether_ntoa (const struct ether_addr *p_a) {
+	static char x[18];
+	return ether_ntoa_r (p_a, x);
+}