net: move ping to sys/net

* moved the former ping module from `sys` to `sys/net/link_layer` and renamed to l2_ping
* use defaulttransceiver instead of cc110x
* some refactoring
net: l2_ping uses radio_packet_t instead of packet_info_t
tests: extend cib unittest
core: inline `cib.c`

The functions in `cib.h` are only used in `msg.c`. They are very small,
and proper function calls have a bigger overhead than inlining these
functions.

The difference in .text size (worst to best):

```
+104 native
+69  qemu-i386
+32  mbed_lpc1768
+28  samr21-xpro

-54  chronos
-54  z1
-56  iot-lab_M3
-56  msb-430
-56  msb-430h
-56  msba2
-56  openmote
-56  spark-core
-56  telosb
-56  wsn430-v1_3b
-56  wsn430-v1_4
-58  arduino-mega2560
-60  airfy-beacon
-60  arduino-due
-60  f4vi1
-60  msbiot
-60  pca10000
-60  pca10005
-60  stm32f0discovery
-60  stm32f3discovery
-60  stm32f4discovery
-60  udoo
-60  yunjia-nrf51822
-72  avsextrem
-72  pttu
-120 redbee-econotag
```
C++ -_-
core: cib size must not exceed MAXINT/2
net: socket: fix for e93d030058684f98894aafb5c4048b2c6d9d479f

The former fix for socket initialization was broken. This fixes the
"fix" by using the right exit condition for the loops.
socket: cosmetics
socket: socket_base_exists_socket returns bool

Adapt the prototype to the actual used return value.
cpu/arm_common: c++: initial support for c++ added _kill (weak) and weak attribute for _kill_r
boards/msba2: provided c++ feature (cpp)
Initial import of a DAC test application
shell: introduced l2 ping shell commands
l2_ping: add to auto_init
travis: check for doxygen warnings

This introduces a script that will check for doxygen warnings in newly
committed or edited files.
travis: added doxygen check script to travis config
Merge pull request #1495 from BytesGalore/EasyCppMixing_msba2_support_fix

cpu/arm_common C++ msba2 support fix
Merge pull request #1228 from OlegHahm/l2_ping

net: Link Layer Ping
Merge pull request #2023 from PeterKietzmann/add_test_periph_dac

test/periph_dac: adds a test for DACs
Merge pull request #2104 from OlegHahm/travis_doccheck

travis: check for doxygen warnings
pr_check: Fix matching

Fixes #2125