core: imply current_prio in `sched_switch()`

There is no need to supply the current priority to `sched_switch()`,
when this function can easily tell the value of
`active_thread->priority` itself.
unittests: Force execution of $(UNITTEST_LIBS) build target
core:debug: documentation for `DEBUG_PRINT` was confusing and wrong
tests: add missing license to test_struct_tm_utility
Merge pull request #1261 from Kijewski/test_struct_tm_utility-license

tests: add missing license to test_struct_tm_utility
pnet: accept should return a new generated file descriptor

From man page:
On success, these system calls return a nonnegative integer that is a
descriptor for the accepted socket. On error, -1 is returned, and errno
is set appropriately.
Merge pull request #1262 from cgundogan/fix_posix_accept

pnet: accept should return a new generated file descriptor
Merge pull request #1057 from Kijewski/sched_switch-current_prio

core: imply current_prio in `sched_switch()`
native: fix coding style
native: set optional path with unix socket option

fixes #1045
Merge pull request #1214 from LudwigOrtmann/issue-1045

native: unix socket path enhancement
Merge pull request #1238 from authmillenon/makefile-unittests

unittests: Force execution of $(UNITTEST_LIBS) build target
x86: fix download script for OSX

Mac OSX does not have `wget`, but `curl`.
It does not understand `tar xJ`, but `tar xj`.
Merge pull request #1250 from Kijewski/x86-osx-fix-download

x86: fix download script for OSX
Merge pull request #1248 from thomaseichinger/debug_h_doc_fix

core:debug: documentation for `DEBUG_PRINT` was confusing and wrong
destiny: segfault due to dereferencing nullpointer
get_socket(i) returns NULL, when no specific socket is found.
Without an appropriate check for NULL, the current state
of the code leads to a segfault.

destiny: added parens

destiny: continuing the loop makes more sense than returning at first sight of NULL
Merge pull request #1269 from cgundogan/fix_get_waiting_connection_socket_null_check

destiny: segfault due to dereferencing nullpointer
core: fix variable name for sched_pidlist
Merge pull request #1272 from OlegHahm/sched_pidlist_fix

core: fix variable name for sched_pidlist
examples: added shell over std-IO usage example