Added -4 -6 Switches and default tcp46

As *BSD defaults to use IPv6 only sockets and Linux to IPv4 mapped IPv6
sockets, switches to support explicit binding address families are required.

Now set explicitly if you want IPv6 only, IPv4 only or mapped IPv4.

Caveat:
OpenBSD explicitly states to not support IPv4 mapped IPv6 via setsock-API

README.md

@@ -19,6 +19,8 @@ Usage information is printed with `-h`.
 
 ```
 Usage: endlessh [-vh] [-d MS] [-f CONFIG] [-l LEN] [-m LIMIT] [-p PORT]
+  -4        Bind to IPv4 only
+  -6        Bind to IPv6 only
   -d INT    Message millisecond delay [10000]
   -f        Set and load config file [/etc/endlessh/config]
   -h        Print this help message and exit
@@ -69,6 +71,12 @@ MaxClients 4096
 #   1 = Standard, useful log messages
 #   2 = Very noisy debugging information
 LogLevel 0
+
+# Set the family of the listening socket
+#   0 = Use IPv4 Mapped IPv6 (Both v4 and v6, default)
+#   4 = Use IPv4 only
+#   6 = Use IPv6 only
+BindFamily 0
 ```
 
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