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.

utils/strbuf.c

@@ -112,3 +112,17 @@ void strbuf_finalise_agent_query(strbuf *buf_o)
     assert(buf->visible.len >= 5);
     PUT_32BIT_MSB_FIRST(buf->visible.u, buf->visible.len - 4);
 }
+
+strbuf *strbuf_dup(ptrlen string)
+{
+    strbuf *buf = strbuf_new();
+    put_datapl(buf, string);
+    return buf;
+}
+
+strbuf *strbuf_dup_nm(ptrlen string)
+{
+    strbuf *buf = strbuf_new_nm();
+    put_datapl(buf, string);
+    return buf;
+}