Mark debug flags which should be implemented differently

I somehow missed the additional SLOW_BEREQ getting in (could be that this was
during my vacation). I think we should avoid adding behavioural changes by
special cased debug flags whenever we have better options, and in this case the
better option are filters.

Which, in this case, would come with #4035, a PR which ironically now needs
additional attention just because of these changes.

include/tbl/debug_bits.h

@@ -41,7 +41,7 @@ DEBUG_BIT(HASHEDGE,		hashedge,	"Edge cases in Hash")
 DEBUG_BIT(VCLREL,		vclrel,		"Rapid VCL release")
 DEBUG_BIT(LURKER,		lurker,		"VSL Ban lurker")
 DEBUG_BIT(ESI_CHOP,		esi_chop,	"Chop ESI fetch to bits")
-DEBUG_BIT(FLUSH_HEAD,		flush_head,	"Flush after http1 head")
+DEBUG_BIT(FLUSH_HEAD,		flush_head,	"Flush after http1 head") // XXX -> filter
 DEBUG_BIT(VTC_MODE,		vtc_mode,	"Varnishtest Mode")
 DEBUG_BIT(WITNESS,		witness,	"Emit WITNESS lock records")
 DEBUG_BIT(VSM_KEEP,		vsm_keep,	"Keep the VSM file on restart")
@@ -52,7 +52,7 @@ DEBUG_BIT(PROCESSORS,		processors,	"Fetch/Deliver processors")
 DEBUG_BIT(PROTOCOL,		protocol,	"Protocol debugging")
 DEBUG_BIT(VCL_KEEP,		vcl_keep,	"Keep VCL C and so files")
 DEBUG_BIT(LCK,			lck,		"Additional lock statistics")
-DEBUG_BIT(SLOW_BEREQ,		slow_bereq,	"Slow down bereq")
+DEBUG_BIT(SLOW_BEREQ,		slow_bereq,	"Slow down bereq")	// XXX -> filter
 #undef DEBUG_BIT
 
 /*lint -restore */