OpenOptionsExt: add flags method

[pfpacket]

I recently encountered the case that needs to open a file with the given raw c_int flags, which is the second argument of open(2).

I think it's better to be able to create a OpenOptions with the given flags in the cases dealing with FFI or network protocols and so on.

Here is my experimental try.
branch: https://github.com/pfpacket/rust/tree/openoptext-flag

Edit:
Now I think from_flag method is unnecessary, so I removed it.

[pfpacket]

rust-lang/rust#30014