Please allow passing options to Dir::open()

[gleon99]

Sometimes options like O_DIRECT are required. Current API doesn't support that.

See also: #1

[cehteh]

I am thinking about some builder pattern here, maybe I can do a 'proof of concept' soon (I'll need control over O_CLOEXEC) as well:

Example:

// Creates a 'Flags' builder
let dir = Dir::new()                            // (defaults only to O_CLOEXEC)
             .with(libc::O_SEARCH)
             .without(libc::O_CLOEXEC)
             .open("somedir");                // Calls back to Dir::open

let subdir =
    dir.without(libc::O_CLOEXEC)  // Flags builder references a Option<Dir> as well
    .subdir("inner");                          // Call back to the Dir

let file = subdir.with(libc::O_DIRECT).open_file("test.txt");
// open_file (and other method) will add the required flags, but leave the rest untouched

Flags are passed as raw libc::c_int and not abstracted in rust and no validated.

1. To allow OS specific flags the implementation doesn't know about
2. Invalid Flags will eventually result in runtime / io errors