sysconf: Implement _SC_ATEXIT_MAX query

and remove the incorrect macro ATEXIT_MAX

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>
Change-Id: Ia5d7dafc50e942f62f95137313c34741c6dd60ba

libs/libc/unistd/lib_sysconf.c

@@ -41,6 +41,7 @@
 #include <nuttx/config.h>
 
 #include <unistd.h>
+#include <sched.h>
 #include <errno.h>
 
 /****************************************************************************
@@ -225,6 +226,13 @@ long sysconf(int name)
       case _SC_OPEN_MAX:
         return CONFIG_NFILE_DESCRIPTORS;
 
+      case _SC_ATEXIT_MAX:
+#ifdef CONFIG_SCHED_EXIT_MAX
+        return CONFIG_SCHED_EXIT_MAX;
+#else
+        return 0;
+#endif
+
       default:
 #if 0 /* Assume valid but not implemented for the time being */
         errcode = EINVAL;