code cleanup: base url detection is incoherent between install time and runtime

Problem/Motivation

drupal_detect_baseurl() contains functionality that looks like it should closely mirror, or be identical to, some code in conf_path(). Rewriting similar or identical code in separate places in drastically different ways degrades legibility and understandability of the codebase overall.

Proposed resolution

If the relevant code is intended to be identical in functionality, ensure it is identical in the codebase.

Remaining tasks

Patch.

User interface changes

None.

API changes

None.

Original report by @Damien Tournoud

In install.php:

function drupal_detect_baseurl($file = 'install.php') {
  global $profile;
  $proto = $_SERVER['HTTPS'] ? 'https://' : 'http://';
  $host = $_SERVER['SERVER_NAME'];
  $port = ($_SERVER['SERVER_PORT'] == 80 ? '' : ':' . $_SERVER['SERVER_PORT']);
  $uri = preg_replace("/\?.*/", '', $_SERVER['REQUEST_URI']);
  $dir = str_replace("/$file", '', $uri);

  return "$proto$host$port$dir";
}

in conf_path():

    // Create base URL
    $base_root = (isset($_SERVER['HTTPS']) && $_SERVER['HTTPS'] == 'on') ? 'https' : 'http';

    // As $_SERVER['HTTP_HOST'] is user input, ensure it only contains
    // characters allowed in hostnames.
    $base_url = $base_root .= '://' . preg_replace('/[^a-z0-9-:._]/i', '', $_SERVER['HTTP_HOST']);

    // $_SERVER['SCRIPT_NAME'] can, in contrast to $_SERVER['PHP_SELF'], not
    // be modified by a visitor.
    if ($dir = trim(dirname($_SERVER['SCRIPT_NAME']), '\,/')) {
      $base_path = "/$dir";
      $base_url .= $base_path;
      $base_path .= '/';
    }
    else {
      $base_path = '/';
    }