Improve POSIX conformance of <signal.h>.

• This header file has always depended on pthread_t, pthread_attr_t, struct timespec, size_t and uid_t. Only as of POSIX 2008, these dependencies have been states explicitly. They should now be defined.

• In our implementation, struct sigevent::sigev_notify_attributes has type "void *" instead of "pthread_attr_t *". My guess is that this was done to prevent pulling in the pthread types, but this can easily be avoided by using the underlying structure types.