CLEANUP: proxy: make the proxy lookup functions more user-friendly

First, findproxy() was renamed proxy_find_by_name() so that its explicit
that a name is required for the lookup. Second, we give this function
the ability to search for tables if needed. Third we now provide inline
wrappers to pass the appropriate PR_CAP_* flags and to explicitly look
up a frontend, backend or table.
diff --git a/src/stream.c b/src/stream.c
index 4d62c71..0f9fbe2 100644
--- a/src/stream.c
+++ b/src/stream.c
@@ -1053,7 +1053,7 @@
 					struct chunk *tmp = get_trash_chunk();
 					if (!build_logline(s, tmp->str, tmp->size, &rule->be.expr))
 						break;
-					backend = findproxy(tmp->str, PR_CAP_BE);
+					backend = proxy_be_by_name(tmp->str);
 					if (!backend)
 						break;
 				}