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
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
Merge remote-tracking branch 'upstream/master' into cbor