add deprecated (removed from posix) [efg]cvt() functions

these have not been heavily tested, but they should work as described
in the old standards. probably broken for non-finite values...

src/stdlib/ecvt.c

@@ -0,0 +1,19 @@
+#include <stdlib.h>
+#include <stdio.h>
+
+char *ecvt(double x, int n, int *dp, int *sign)
+{
+	static char buf[16];
+	char tmp[32];
+	int i, j;
+
+	if (n-1U > 15) n = 15;
+	sprintf(tmp, "%.*e", n-1, x);
+	i = *sign = (tmp[0]=='-');
+	for (j=0; tmp[i]!='e'; j+=(tmp[i++]!='.'))
+		buf[j] = tmp[i];
+	buf[j] = 0;
+	*dp = atoi(tmp+i+1);
+
+	return buf;
+}