Rewrite some manual char-buffer-handling code.

In the course of recent refactorings I noticed a couple of cases where
we were doing old-fashioned preallocation of a char array with some
conservative maximum size, then writing into it via *p++ or similar
and hoping we got the calculation right.

Now we have strbuf and dupcat, so we shouldn't ever have to do that.
Fixed as many cases as I could find by searching for allocations of
the form 'snewn(foo, char)'.

Particularly worth a mention was the Windows GSSAPI setup code, which
was directly using the Win32 Registry API, and looks much more legible
using the windows/utils/registry.c wrappers. (But that was why I had
to enhance them in the previous commit so as to be able to open
registry keys read-only: without that, the open operation would
actually fail on this key, which is not user-writable.)

Also unix/askpass.c, which was doing a careful reallocation of its
buffer to avoid secrets being left behind in the vacated memory -
which is now just a matter of ensuring we called strbuf_new_nm().

unix/dialog.c

@@ -594,9 +594,7 @@ void dlg_listbox_addwithid(dlgcontrol *ctrl, dlgparam *dp,
         cols = cols ? cols : 1;
         for (i = 0; i < cols; i++) {
             int collen = strcspn(text, "\t");
-            char *tmpstr = snewn(collen+1, char);
-            memcpy(tmpstr, text, collen);
-            tmpstr[collen] = '\0';
+            char *tmpstr = mkstr(make_ptrlen(text, collen));
             gtk_list_store_set(uc->listmodel, &iter, i+1, tmpstr, -1);
             sfree(tmpstr);
             text += collen;