Thanks to John Cummings.

src/cmd/upas/unesc/unesc.c

@@ -0,0 +1,48 @@
+/*
+ *	upas/unesc - interpret =?foo?bar?=char?= escapes
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+
+int
+hex(int c)
+{
+	if('0' <= c && c <= '9')
+		return c - '0';
+	if('A' <= c && c <= 'F')
+		return c - 'A' + 10;
+	if('a' <= c && c <= 'f')
+		return c - 'a' + 10;
+	return 0;
+}
+
+void
+main(int argc, char **argv)
+{
+	int c;
+
+	while((c=getchar()) != EOF){
+		if(c == '='){
+			if((c=getchar()) == '?'){
+				while((c=getchar()) != EOF && c != '?')
+					continue;
+				while((c=getchar()) != EOF && c != '?')
+					continue;
+				while((c=getchar()) != EOF && c != '?'){
+					if(c == '='){
+						c = hex(getchar()) << 4;
+						c |= hex(getchar());
+					}
+					putchar(c);
+				}
+				(void) getchar();	/* consume '=' */
+			}else{
+				putchar('=');
+				putchar(c);
+			}
+		}else
+			putchar(c);
+	}
+	exit(0);
+}