Merge pull request #4321 from kaspar030/cleanup_posix

Makefile.dep

cpu/atmega_common/avr-libc-extra/unistd.h

cpu/msp430-common/include/unistd.h

sys/include/posix_io.h

sys/posix/fd.c

sys/posix/include/unistd.h

sys/posix/posix_io.c

sys/posix/unistd.c

sys/xtimer/xtimer_posix.c

tests/posix_sleep/Makefile

tests/posix_sleep/main.c

tests/pthread/Makefile

tests/pthread_cooperation/Makefile

tests/pthread_tls/Makefile