The VFS TAP plugin offers access to Genode's Uplink by providing
a special file system. It exposes a data file that reflects a _/dev/tap0_
file. The support of I/O control operations is provided in form of a structured
'info' file located in the directory named after the data file, e.g.
_/dev/.tap0/info_.

This file may by used to query the configured parameters and has the following
structure:

! <tap name="tap0" mac_addr="..."/>

Each parameter can also be accessed via its own file. The following list
presents all files:

  * :mac_addr (rw): The MAC address of the device (immutable when in Nic mode).
  * :name (ro):     The name of the device.

When mounting the tap file system, the following optional attributes may
be provided:

  * :label:  Sets the session label of the Uplink/Nic session. If not provided,
             an empty label is used.
  * :mac:    Sets the default mac address.

The following config snippet illustrates its configuration:

! <vfs>
!   <dir name="dev">
!     <tap name="tap0"/>
!   </dir>
! </vfs>

Note, that the plugin emulates the tap device and its I/O control operations
as expected by FreeBSD's libc. On Linux, the tap devices are created by
performing an I/O control operation on _/dev/net/tun_ after which the opened
file descriptor can be used for reading/writing. If only a single tap device is
needed, it is possible to use the tap plugin for _/dev/net/tun_ and just omit
the ioctl by creating an emulation header file _linux/if_tun.h_ with the
following content:

! #include <net/if_tap.h>
! #define TUNSETIFF TAPGIFNAME
! #define IFF_TAP   0
! #define IFF_NO_PI 0
