MAJOR: sample: don't pass l7 anymore to sample fetch functions

All of them can now retrieve the HTTP transaction *if it exists* from
the stream and be sure to get NULL there when called with an embryonic
session.

The patch is a bit large because many locations were touched (all fetch
functions had to have their prototype adjusted). The opportunity was
taken to also uniformize the call names (the stream is now always "strm"
instead of "l4") and to fix indent where it was broken. This way when
we later introduce the session here there will be less confusion.
diff --git a/include/types/sample.h b/include/types/sample.h
index e054c27..ce03d9e 100644
--- a/include/types/sample.h
+++ b/include/types/sample.h
@@ -285,8 +285,7 @@
 struct sample_fetch {
 	const char *kw;                           /* configuration keyword */
 	int (*process)(struct proxy *px,
-	               struct stream *l4,
-	               void *l7,
+	               struct stream *strm,
 		       unsigned int opt,          /* fetch options (SMP_OPT_*) */
 		       const struct arg *arg_p,
 	               struct sample *smp,