This reverts commit 69c526f4.

Instead of removing ENABLE_DEBUG, define it as zero and replacing the
ifdef preprocessor commands by a simple #if

* some cosmetics and cleanups

changed types irq_callback, hashtable, posix_io, s_display_flags, seq_buffer_entry, tcb, toprint to type_t"

* msg_send_int now sets sender_pid to receiver_pid