replace opathdir with opendir_flags

replace opathdir with opendir_flags

`opathdir` was used to replace `opendir` in order to ensure
`O_NOFOLLOW` and `O_CLOEXEC` were set, however it also added
`O_PATH` which prevents `readdir`/`getdents` to be used on
it, causing the `/sys/devices/system/cpu/<subdir>`
directories to be empty.

Instead, let's have an `opendir_flags` utility which simply
passed additional flags to the `open(..., O_DIRECTORY)` call
preceding `fdopendir()`.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>