Add "path" in phpdoc of connections supporting UNIX domain sockets.
Improve URI parsing for connection parameters.

Using PHP's "parse_str()" to parse the query string is slightly more
efficient then our own code especially when the number of fields in
the query string grows, with the additional benefit of supporting
arrays for values when brackets are present in fieldnames.

So after this commit, providing this URI string:

  $string = 'tcp://127.0.0.1?metavars[]=foo&metavars[]=hoge';

Is equivalent to providing the following named array:

  $array = [
    'scheme' => 'tcp',
    'host' => '127.0.0.1',
    'metavars' => ['foo', 'hoge'],
  ];

Other improvements are that parsing does not break when the value of a
field contains one or more "=" and empty or incomplete "key=value"
pairs result in an empty string for "key".