use internal malloc for posix_spawn file actions objects

this makes it possible to perform actions on file actions objects with
a libc-internal lock held without creating lock order relationships
that are silently imposed on an application-provided malloc.

src/process/fdop.h

@@ -10,3 +10,8 @@ struct fdop {
 	mode_t mode;
 	char path[];
 };
+
+#define malloc __libc_malloc
+#define calloc __libc_calloc
+#define realloc undef
+#define free __libc_free