CLEANUP: Use the ist() macro whenever possible

Refactoring performed with the following Coccinelle patch:

    @@
    char *s;
    @@

    (
    - ist2(s, strlen(s))
    + ist(s)
    |
    - ist2(strdup(s), strlen(s))
    + ist(strdup(s))
    )

Note that this replacement is safe even in the strdup() case, because `ist()`
will not call `strlen()` on a `NULL` pointer. Instead is inserts a length of
`0`, effectively resulting in `IST_NULL`.
diff --git a/src/http_ana.c b/src/http_ana.c
index 5536c04..c124544 100644
--- a/src/http_ana.c
+++ b/src/http_ana.c
@@ -2733,7 +2733,7 @@
 	/* Do we have a custom reason format string? */
 	if (!isttest(reason)) {
 		const char *str = http_get_reason(status);
-		reason = ist2(str, strlen(str));
+		reason = ist(str);
 	}
 
 	if (!http_replace_res_status(htx, ist2(trash.area, trash.data), reason))