time: Introduce function to check correctness of nanoseconds value

The valid_nanoseconds () static inline function has been introduced to
check if nanoseconds value is in the correct range - greater or equal to
zero and less than 1000000000.

The explicit #include <time.h> has been added to files where it was
missing.

The __syscall_slong_t type for ns has been used to avoid issues on x32.

Tested with:
- scripts/build-many-glibcs.py
- make PARALLELMFLAGS="-j12" && make PARALLELMFLAGS="-j12" xcheck on x86_64