(WSTOPPED, WEXITED, WCONTINUED, WNOWAIT): New macros.
* sysdeps/unix/sysv/linux/kernel-features.h (__ASSUME_WAITID_SYSCALL):
New macro.
* sysdeps/unix/sysv/linux/waitid.c: New file. Use new syscall when
available, or fall back to the waitpid-based generic code.
