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/pty.c

@@ -1228,9 +1228,8 @@ Backend *pty_backend_create(
             char *shellname;
             if (conf_get_bool(conf, CONF_login_shell)) {
                 const char *p = strrchr(shell, '/');
-                shellname = snewn(2+strlen(shell), char);
                 p = p ? p+1 : shell;
-                sprintf(shellname, "-%s", p);
+                shellname = dupprintf("-%s", p);
             } else
                 shellname = (char *)shell;
             execl(shell, shellname, (void *)NULL);