MINOR: sample: fill the struct sample with the options.

Options are relative to the sample. Each sample fetched is associated with
fetch options or fetch flags.

This patch adds the 'opt' vaue in the sample struct. This permits to reduce
the sample-fetch function prototype. In other way, the converters will have
more detail about the origin of the sample.
diff --git a/src/sample.c b/src/sample.c
index 8cc2234..353d6bb 100644
--- a/src/sample.c
+++ b/src/sample.c
@@ -1036,6 +1036,7 @@
 	p->px   = px;
 	p->sess = sess;
 	p->strm = strm;
+	p->opt  = opt;
 	if (!expr->fetch->process(opt, expr->arg_p, p, expr->fetch->kw, expr->fetch->private))
 		return NULL;