make: del no-delete-null-pointer-checks for clang++

This PR enables the use of clang++ to compile C++ projects by adding
`CXXUWFLAGS += -fno-delete-null-pointer-checks`, if `${CC}` understands
`-fno-delete-null-pointer-checks` but `${CXX}` does not.
native/freebsd: fix tapsetup permission
native/freebsd: fix usecond typdef
Merge pull request #1729 from Kijewski/clang++

make: del no-delete-null-pointer-checks for clang++
nativenet: Apply coding conventions
nativenet: Supply net_dev_t support for nativenet
netdev.tests: Remove unneccessary #ifndef path
Merge pull request #1732 from authmillenon/netdev-nativenet

nativenet: Port nativenet for netdev
Merge pull request #1780 from LudwigOrtmann/native-freebsd-fixup

sys/posix: FreeBSD fixup
sixlowpan: fix ipv6_addr_init_prefix