ports: add openvpn-2.3.4

The port is succesfully tested with echo test, lighttpd and
rdesktop. DHCP over OpenVPN is not tested, therefore the ip address etc.
needs to be specified manually.
openssl: use Genode specific RAND backend

For now, we merely provide "/dev/random".
openssl: update to version 1.0.1i
libports: add jitterentropy-20140411
vfs: add vfs handle header to single_file header
libc: add support for external vfs file systems

These file systems are provided on-demand by loading a shared library
when the fstab node is traversed. By convention this library is named
after the file system it provides. For example a file system that
provides a 'random' file system node is called 'vfs_random.lib.so'. It
is still possible to give the the node another name in the vfs. The
following code snippts illustrates this matter:

! [...]
! <config>
!   <libc>
!     <vfs>
!       <dir name="dev"> <jitterentropy name="random"/> </dir>
!     </vfs>
!   </libc>
! </config>
! [...]

Here the jitterentropy file system, implemented in
'vfs_jitterentropy.lib.so' provides a file system node named 'random'
in the 'dev' directory. When traversing the vfs section the libc will
try to load 'vfs_jitterentropy.lib.so' but programs may access the
file system only via '/dev/random'.
libports: add vfs_jitterentropy library