Implement initialization, cleanup and mutex struct

Implement initialization, cleanup and mutex struct

The mutex struct is implemented because pthread_mutex_destroy
produces undefined behaviour when supplied with uninitialized mutex.
The struct holds information whether the mutex has been initialized
succesfully.

The header file philo.h has also been filled with declarations of
functions used in main so that Makefiles compiles every file to object.
This is done for easier checking of compile errors.