Implement wait4 (syscall for waitpid, wait) about shadow

[sporksmith]

Some notes on the status integer set by wait4 etc.

Theoretically this is part of the kernel ABI, but I haven't been able to find anything referencing this integer in the kernel user-space headers or ABI docs.

glibc's waitstatus.h has macros for examining the status integer with decent comments, and macros for constructing this status. Not as authoritative as the kernel code, but a pretty useful hint:

/* Macros for constructing status values.  */
#define	__W_EXITCODE(ret, sig)	((ret) << 8 | (sig))
#define	__W_STOPCODE(sig)	((sig) << 8 | 0x7f)
#define __W_CONTINUED		0xffff
#define	__WCOREFLAG		0x80

The kernel code for constructing the status is a bit scattered around https://github.com/torvalds/linux/blob/89bf6209cad66214d3774dac86b6bbf2aec6a30d/kernel/exit.c. EDIT: the specific lines pasted here before were wrong/misleading; the __W_EXITCODE macro above tells us what we need to know for exited children.