Save and restore oa_present bitfield

Implements most of #17, except that we would like to restore OA_VARY
with the log read...

This should be backwards and forwards compatible, because the default
in both cases is 0, and varnish-cache will not use the bitfield
if it is 0.

 Conflicts:
	src/fellow_cache.c

 because fellow_disk_obj_compat() in master but not in 7.3

src/fellow_cache.c

@@ -448,8 +448,9 @@ struct fellow_disk_obj {
 #define FDO_FIXATTR(U, l, s, vs)			\
 	uint8_t			fa_##l[s];
 #include "tbl/fellow_obj_attr.h"
-	uint8_t			fa_reserve[64];
+	uint8_t			fa_reserve[62];
 
+	uint16_t			fdoa_present;
 #define FDO_VARATTR(U, l)			\
 	struct fellow_disk_obj_attr	va_##l;
 #include "tbl/fellow_obj_attr.h"
@@ -3857,6 +3858,7 @@ fellow_busy_done(struct fellow_busy *fbo, struct objcore *oc, unsigned inlog)
 	if (inlog) {
 		fdo->fdo_flags |= FDO_F_INLOG;
 		fco->oc = oc;
+		fdo->fdoa_present = oa2fdoa_present(oc->oa_present);
 	}
 	fellow_cache_obj_unbusy(fbo, inlog ? FCOL_WANTLOG : FCOL_NOLOG);
 }
@@ -4167,6 +4169,10 @@ fellow_cache_obj_get(struct fellow_cache *fc,
 	if (err != NULL)
 		goto err;
 
+	// or because oa_inlog_bit
+	if (fco->oc)
+		fco->oc->oa_present |= fdoa2oa_present(fdo->fdoa_present);
+
 	fdsl = (void *)((uint8_t *)fdo + sizeof *fdo + fdo->va_data_len);
 	assert(PAOK(fdsl));
 	// XXX load of folow-up seglists could be async