core: Fix msg_send_receive

Fixes #1935
pktbuf: optimize size overhead in packet buffer
Merge pull request #1959 from authmillenon/pktbuf-opt

pktbuf: optimize size overhead in packet buffer
unittests: Add missing semicolons to macro calls
make: remove downloader "axel"

Breaks for `make -B`. If the file already exists, then `axel` fails
because it does not find the status file for the download.
Merge pull request #1929 from Kijewski/no-axel

make: remove downloader "axel"
Merge pull request #1972 from authmillenon/unittests-semicolons

unittests: Add missing semicolons to macro calls
unittests: make test macros save for strict aliasing
Merge pull request #1973 from authmillenon/test-macros

embunit: make test macros save for strict aliasing
c++: sys: add extern C to header files
Merge pull request #1793 from BytesGalore/extern_C_in_headers_sys

sys: add extern "C" to headers
sixlowpan: rename to sixlowpan_legacy
[SQUASH ME] same treatment for ipv6.h
[SQUASH ME] also for the macros
Merge branch 'sixlowpan_legacy' into '6lowpan-fix-major'
Merge branch 'issue-1935' into '6lowpan-fix-major'
ipv6: Initial import of preliminary refactored IPv6 header
sixlowpan: refactor 6LoWPAN fragmentation for netapi
sixlowpan.iphc_cbuf: Initial import of an IPHC context buffer
sixlowpan: Initial import of refactored IPHC