breaking: Fix fsys.OpenFile confusion of flags and permissions

[kke]

OpenFile used to accept FileMode twice, as in:

func (f *PosixFsys) OpenFile(path string, mode FileMode, perm FileMode)
  (*PosixFile, error)

The correct (or stdlib equivalent) signature is to accept an integer for the status flags and fs.FileMode for the permissions, as in:

func (f *PosixFsys) OpenFile(path string, flags int, perm fs.FileMode)
  (*PosixFile, error)

The status flags are:

File access:

• O_RDONLY - open the file read-only. The value is 0x0, you can pass a zero if you just want to read a file (this is what fsys.Open does and it should be used instead for that purpose.).
• O_WRONLY - open the file write-only.
• O_RDWR - open the file read-write.
• O_SYNC is implied, all operations are synchronous.

File creation:

• O_APPEND - append data to the file when writing.
• O_CREATE - create a new file if none exists.
• O_TRUNC - truncate the file if it exists
• O_EXCL - used in conjunction with O_CREATE, file must not already exist.

Usage:

// Explanation of the flags:
// - Create a new file if one doesn't exist
// - Truncate an existing file if one exist (keeps
//   existing permissions)
// - Open the file for writing
fsys.OpenFile("/tmp/example", rigfs.O_CREATE|rigfs.O_TRUNC|rigfs.O_WRONLY,
    fs.FileMode(0644))

The rigfs.FileMode was retired, you're now expected to use fs.FileMode from "io/fs".

There were some other omissions and logical errors in the previous implementation of the function.