libports: add vfs_jitterentropy library

This commit adds a port the jitterentropy library to Genode. As backend
on x86_{32,64} 'rdtsc' is used and on ARMv{6,7} the performance-counter.

Fixes #1239.
libc: add if_tun.h

Fixes #1235.
ports: add openvpn-2.3.4

The port was succesfully tested a echo test and lighttpd. DHCP over
OpenVPN is not tested and probably will not work out of the box.
Therefore, the ip address etc. need to be specified manually.

For now, only ethernet bridging (using a TAP device) is supported.

Fixes #1235.
libc: add net/route.h and its dependencies

Fixes #1235.