provide a helper function for block delta updates

just a note that the patch doesn't currently leverage the new function for the core block updates, but it easily could -- and i'm happy to add that if people think this function is a good idea... :)

the first patch wasn't quite complete, and fixing it convinces me even more that a helper function is needed here, since properly doing this update is a bit involved -- if you examine the existing system_update_7004() you'll see what i'm talking about.

the new approach basically steals the relevant code from system_update_7004() and moves it to update.inc, thus making it available as a helper function to any modules that need it.

with the helper function, a contrib module's process for updating it's blocks would look like this:

function mymodule_update_7XXX(&$sandbox) {
  $renamed_deltas = array(
    'mymodule' => array(
      '0' => 'mymodule-block-1',
      '1' => 'mymodule-block-2',
    ),
  );
  update_fix_block_deltas($sandbox, $renamed_deltas);

  // This conditional would be optional.
  if (!isset($sandbox['progress'])) {
    // Anything they may want to do just once during the update...
  }
}

there was a small logic error in my example function in #2, and my adjustments to system_update_7004() to leverage the helper function. any one-time updates need to be run *before* update_fix_block_deltas() is run, as it sets $sandbox['progress'] the first time it's run.

attached patch fixes system_update_7004() to account for this -- it's a simple copy/paste of the code from one side of the helper function to the other, no other changes have been made.

and here's an updated example function:

function mymodule_update_7XXX(&$sandbox) {
  $renamed_deltas = array(
    'mymodule' => array(
      '0' => 'mymodule-block-1',
      '1' => 'mymodule-block-2',
    ),
  );

  // This conditional would be optional.
  if (!isset($sandbox['progress'])) {
    // Anything they may want to do just once during the update...
  }

  update_fix_block_deltas($sandbox, $renamed_deltas);
}

added this to #563106: Cannot upgrade from Drupal 6 to Drupal 7 - meta issue -- this is really a no brainer to ease the upgrade process, especially given all the D7UX promises.

re-rolled and updated based on webchick's requests in #10

in what might be the most insignificant patch of the D7 release cycle, this fixes a bad semicolon in the example code in the code comments for update_fix_d7_block_deltas().