Use signals for delivering input events

This patch changes both the Input::Session interface and the skeleton
for the server-side implementation of this interface
('input/component.h').

The Input::Session interface offers a new 'sigh' function, which can be
called be the client to register a signal handler. The signal handler
gets notified on the arrival of new input. This alleviates the need to
poll for input events at the client side.

The server-side skeleton for implementing input services underwent a
redesign to make it more modular and robust. I.e., there are no
global functions needed at the server side and the event-queue
enable/disable mechanism is implemented at a central place (in the root
component) rather than inside each driver.

Fixes #46
dde_linux: implement memory barriers in USB driver

This patch implements the memory barrier macros in the USB driver for ARMv7
and x86.

Fixes #1159.
ram_blk: change blocksize output to decimal

Reading 200 instead of 512 is confusing.

Fixes #1142.
ram_blk: remove superfluous block_number check

block_number is Block::sector_t (uint64_t), which never can by less
than 0.

Fixes #1142.
Qt5: use the Genode main thread

Now that it is possible to resize the stack of the Genode main thread, it
is not necessary anymore to create a new Genode thread as Qt main thread.

Fixes #1134.
Qt5: constrain the window buffer refresh area

With this patch, the information about the changed window area is taken
into account when refreshing the Nitpicker buffer.

Fixes #1132.
linux: fix lx_rmap test do not stuck

The test did not fail actually. It just behaved strangely and got stuck
in the good case, which is fixed now.

Fixes #1121.
libports: add pkcs11-helper-nl-1.10
libports: add polarssl-nl-1.2.10
libports: add if_tun.h to libc
ports: add openvpn-nl-2.3.2 [WIP]

Openvpn compiles but tun(4) is still not implemented and it needs
<net/if_tun.h> (manually symlinked because a recent port handling
changes.