MINOR: tools: make str2sa_range() directly return the protocol

We'll need this so that it can return pointers to stacked protocol in
the future (for QUIC). In addition this removes a lot of tests for
protocol validity in the callers.

Some of them were checked further apart, or after a call to
str2listener() and they were simplified as well.

There's still a trick, we can fail to return a protocol in case the caller
accepts an fqdn for use later. This is what servers do and in this case it
is valid to return no protocol. A typical example is:

   server foo localhost:1111

include/haproxy/tools.h

@@ -241,10 +241,9 @@ static inline int is_idchar(char c)
  * If <pfx> is non-null, it is used as a string prefix before any path-based
  * address (typically the path to a unix socket).
  */
-struct sockaddr_storage *str2sa_range(const char *str,
-                                      int *port, int *low, int *high, int *fd,
-                                      char **err, const char *pfx,
-                                      char **fqdn, unsigned int opts);
+struct sockaddr_storage *str2sa_range(const char *str, int *port, int *low, int *high, int *fd,
+                                      struct protocol **proto, char **err,
+                                      const char *pfx, char **fqdn, unsigned int opts);
 
 /* converts <str> to a struct in_addr containing a network mask. It can be
  * passed in dotted form (255.255.255.0) or in CIDR form (24). It returns 1