[MINOR] add the splice_len member to the buffer struct in preparation of splice support

In preparation of splice support, let's add the splice_len member
to the buffer struct. An earlier implementation made it conditional,
which made the whole logics very complex due to a large number of
ifdefs.

Now BF_EMPTY is only set once both buf->l and buf->splice_len are
null. Splice_len is initialized to zero during buffer creation and
is currently not changed, so the whole logics remains unaffected.

When splice gets merged, splice_len will reflect the number of bytes
in flight out of the buffer but not yet sent, typically in a pipe for
the Linux case.
diff --git a/include/types/buffers.h b/include/types/buffers.h
index f15d33d..b310aed 100644
--- a/include/types/buffers.h
+++ b/include/types/buffers.h
@@ -127,6 +127,7 @@
 	int wto;                        /* write timeout, in ticks */
 	int cto;                        /* connect timeout, in ticks */
 	unsigned int l;                 /* data length */
+	unsigned int splice_len;        /* number of bytes remaining in splice, out of buffer */
 	char *r, *w, *lr;               /* read ptr, write ptr, last read */
 	char *rlim;                     /* read limit, used for header rewriting */
 	unsigned int send_max;          /* number of bytes the sender can consume */