[util] Add os_dirent for device enumeration

This is used to enumerate physical devices on Fuchsia
as we migrate away from depending on libfdio.

The implementation uses the Fuchsia specific callback
to gain access to the directory in the namespace.

Bug: 13095

Change-Id: Icb4b898735b033b6feea88c6fa9040401dcd4a39
14 files changed