File creation behavior

[nclack]

closes #32

Adds a test, and modifies create_file() implementations across supported platforms so that they exhibit the following behavior:

1. When creating a file for a path that already exists, creation should succeed, truncating the original file.
2. When creating a file (for writing) that has already been opened for the process, creation should fail, and the original file should be unaffected.

• windows - CreateFile api just does what we want. That one was already working.
• osx - has a special flag we can pass to open that does what we want.
• linux - use the flock api. According to man 2 flock this has been a syscall since kernel 2.0. Potentially racy, but that's probably ok.

[andy-sweet]

After checking the relevant posix docs, looks good to me.