posix: add semaphore implementation
remove sixlowpans own semaphore and use new riot semaphore in flowcontrol
fix spelling in msg.c
harmonize msg.c DEBUG statements

Use same order (function: [thread_name:] message) throughout msg.c.
Begin message with a capital letter.
explain the "brainfuck condition"
fix spelling in msg.h
document msg_init_queue return values
Merge pull request #218 from mehlis/semaphore

posix: add semaphore
Merge pull request #275 from LudwigOrtmann/fix_msg_spelling

fix spelling and harmonize DEBUG statements in msg.[ch]