kbx: Use custom estream buffering

* kbx/keybox-init.c (ll_buffer_size): New var intialized to 128k
(stream_buffers): New var.
(keybox_set_buffersize): New.
(_keybox_ll_open, _keybox_ll_close): Implement buffering.

* sm/gpgsm.c (oKbxBufferSize): New.
(opts): Add option --kbx-buffer-size.
(main): Call keybox_set_buffersize.

* g10/gpg.c: Include keybox.h.
* (oKbxBufferSize): New.
(opts): Add option --kbx-buffer-size.
(main): Call keybox_set_buffersize.
--

Commit message from 2.4:

Running a test on Windows using a pubring.kbx with
Total number of blobs:     2098
              openpgp:     1294
                 x509:      803
and a size of 42MiB with

gpgsm -k --with-validation --disable-dirmngr --kbx-buffer-size N >nul

gives these performance figures using procmon

| N(k) | file events | time(s) |
|------+-------------+---------|
|    0 |     4900000 |      86 |
|   16 |     2456000 |      58 |
|   32 |     1233000 |      43 |
|   64 |      622000 |      37 |
|  128 |      317000 |      32 |
|  256 |      164000 |      31 |
|  512 |       88000 |      30 |

Using _open instead of CreateFile give the same number of file events
but increased the time slight by one second for the measured buffer
size of 64k and 128k.   Benchmarks for gpg have not been conducted.

sm/gpgsm.c

@@ -201,6 +201,7 @@ enum cmd_and_opt_values {
   oIgnoreCertWithOID,
   oRequireCompliance,
   oCompatibilityFlags,
+  oKbxBufferSize,
   oNoAutostart
  };
 
@@ -424,6 +425,7 @@ static ARGPARSE_OPTS opts[] = {
   ARGPARSE_s_s (oLCmessages, "lc-messages", "@"),
   ARGPARSE_s_s (oXauthority, "xauthority", "@"),
   ARGPARSE_s_s (oCompatibilityFlags, "compatibility-flags", "@"),
+  ARGPARSE_p_u (oKbxBufferSize,  "kbx-buffer-size", "@"),
 
   ARGPARSE_header (NULL, ""),  /* Stop the header group.  */
 
@@ -1434,6 +1436,10 @@ main ( int argc, char **argv)
 
         case oRequireCompliance: opt.require_compliance = 1;  break;
 
+        case oKbxBufferSize:
+          keybox_set_buffersize (pargs.r.ret_ulong, 0);
+          break;
+
         default:
           if (configname)
             pargs.err = ARGPARSE_PRINT_WARNING;