BUG/MINOR: server: report correct error message for invalid port on "socks4"

BUG/MINOR: server: report correct error message for invalid port on "socks4"

The socks4 keyword parser was a bit too much copy-pasted, it only checks
for a null port and reports "invalid range". Let's properly check for the
1-65535 range and report the correct error.

It may be backported everywhere "socks4" is present (2.0).

(cherry picked from commit 9743f709d03c036b7a6da2fb260d0e77f9e04a3d)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit e96489ea5e0408d852fc1dd4318926923916c230)
Signed-off-by: Willy Tarreau <w@1wt.eu>