libbb: two new functions: wait_for_exitstatus(pid), xfchdir(fd)

Bartosz Golaszewski proposed xfchdir()

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

libbb/xfuncs.c

@@ -315,3 +315,15 @@ int FAST_FUNC wait4pid(pid_t pid)
 		return WTERMSIG(status) + 0x180;
 	return 0;
 }
+
+// Useful when we do know that pid is valid, and we just want to wait
+// for it to exit. Not existing pid is fatal. waitpid() status is not returned.
+int FAST_FUNC wait_for_exitstatus(pid_t pid)
+{
+	int exit_status, n;
+
+	n = safe_waitpid(pid, &exit_status, 0);
+	if (n < 0)
+		bb_perror_msg_and_die("waitpid");
+	return exit_status;
+}