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.

sshpubk.c

@@ -199,8 +199,7 @@ static int rsa1_load_s_internal(BinarySource *src, RSAKey *key, bool pub_only,
         if (enclen & 7)
             goto end;
 
-        buf = strbuf_new_nm();
-        put_datapl(buf, get_data(src, enclen));
+        buf = strbuf_dup_nm(get_data(src, enclen));
 
         unsigned char keybuf[16];
         hash_simple(&ssh_md5, ptrlen_from_asciz(passphrase), keybuf);