As of POSIX:2008, the waitid() function and its related symbolic constants have been moved to the Base specification. The relevant declarations are already in core/sys/posix/sys/wait.d, but are commented out. Please uncomment them -- I'd like to use waitid() in the new std.process.
Reference:
http://www.opengroup.org/onlinepubs/9699919799/basedefs/sys_wait.h.html