Utility function strbuf_dup.

If you already have a string (of potentially-binary data) in the form
of a ptrlen reference to somewhere else, and you want to keep a copy
somewhere, it's useful to copy it into a strbuf. But it takes a couple
of lines of faff to do that, and it's nicer to wrap that up into a
tiny helper function.

This commit adds that helper function strbuf_dup, and its non-movable
sibling strbuf_dup_nm for secret data. Also, gone through the existing
code and found a bunch of cases where this makes things less verbose.

misc.h

@@ -52,6 +52,10 @@ struct strbuf {
 strbuf *strbuf_new(void);
 strbuf *strbuf_new_nm(void);
 
+/* Helpers to allocate a strbuf containing an existing string */
+strbuf *strbuf_dup(ptrlen string);
+strbuf *strbuf_dup_nm(ptrlen string);
+
 void strbuf_free(strbuf *buf);
 void *strbuf_append(strbuf *buf, size_t len);
 void strbuf_shrink_to(strbuf *buf, size_t new_len);