Add new b_is_multisite() function

[ghost]

There are 3 possibilities when running a b command on a Backdrop site:

1. It's a regular, non-multisite setup
2. It's a multisite setup and you know the site
3. It's a multisite setup and you don't know the site

In the first situation, any site-specific commands/information should be possible as the 'site' is the root dir of Backdrop (e.g. /settings.php).

In the second situation, you're either running the command from the site dir, or you specified the --url option, so the site is known. Site-specific commands/info are possible (e.g. the site is /sites/[site]/settings.php).

In the third situation, you're running the command from outside the site dir (e.g. Backdrop root) and you didn't specify the --url option, so the site is unknown. Site-specific commands/info are not possible (e.g. we don't know the site dir to use to find settings.php, and we can't use the root settings.php as it'll be empty).

Currently, situations 1 and 3 are indistinguishable as BACKDROP_SITE is not set in either case (either because it's the same as BACKDROP_ROOT or because we don't know what it is). This is problematic as we can't provide site-specific info for the first situation when we otherwise should be.

So I propose a new b_is_multisite() function that looks something like this:

function b_is_multisite($root) {
  $sites_php = $root . '/sites/sites.php';
  if (file_exists($sites_php)) {
    $sites = array();
    include $sites_php;
    if (!empty($sites)) {
      return TRUE;
    }
  }

  return FALSE;
}

This will allow us to distinguish between situations 1 and 3 and provide appropriate commands/info as necessary.

[ghost]

I've committed my PR. As we find a use for it we can start using it in the code.