Add a hint in the accept_filter documentation

Trying to understand why I was seeing setsockopt fail on FreeBSD VTEST
boxes and why it failed with an undocumented ENOENT I finally realized
that I needed to kldload accf_http beforehand.

The ENOENT errno was probably a result of accept_filter code not finding
a kernel module for "httpready".

include/tbl/params.h

@@ -48,7 +48,8 @@ PARAM(
 	/* units */	"bool",
 	/* flags */	XYZZY,
 	/* s-text */
-	"Enable kernel accept-filters.",
+	"Enable kernel accept-filters. This may require a kernel module to "
+	"be loaded to have an effect when enabled.",
 	/* l-text */	NULL,
 	/* func */	NULL
 )

lib/libvarnish/vtcp.c

@@ -153,7 +153,6 @@ VTCP_filter_http(int sock)
 
 	memset(&afa, 0, sizeof afa);
 	bprintf(afa.af_name, "%s", "httpready");
-	errno = 0;
 	retval = setsockopt(sock, SOL_SOCKET, SO_ACCEPTFILTER,
 	    &afa, sizeof afa);
 	return (retval);