CLEANUP: Use ist2(const void*, size_t) whenever possible

Refactoring performed with the following Coccinelle patch:

    @@
    struct ist i;
    expression p, l;
    @@

    - i.ptr = p;
    - i.len = l;
    + i = ist2(p, l);
diff --git a/src/51d.c b/src/51d.c
index f36f5fa..6cd1d28 100644
--- a/src/51d.c
+++ b/src/51d.c
@@ -242,8 +242,8 @@
 	ALREADY_CHECKED(htx);
 
 	for (i = 0; i < global_51degrees.header_count; i++) {
-		name.ptr = (global_51degrees.header_names + i)->area;
-		name.len = (global_51degrees.header_names + i)->data;
+		name = ist2((global_51degrees.header_names + i)->area,
+			    (global_51degrees.header_names + i)->data);
 		ctx.blk = NULL;
 
 		if (http_find_header(htx, name, &ctx, 1)) {
@@ -281,8 +281,8 @@
 	ALREADY_CHECKED(htx);
 
 	for (i = 0; i < global_51degrees.header_count; i++) {
-		name.ptr = (global_51degrees.header_names + i)->area;
-		name.len = (global_51degrees.header_names + i)->data;
+		name = ist2((global_51degrees.header_names + i)->area,
+			    (global_51degrees.header_names + i)->data);
 		ctx.blk = NULL;
 
 		if (http_find_header(htx, name, &ctx, 1)) {