Document EINVAL errno as documented in:

http://www.opengroup.org/onlinepubs/009695399/functions/bind.html

Index: lib/libc/sys/bind.2
===================================================================
--- lib/libc/sys/bind.2	(wersja 192959)
+++ lib/libc/sys/bind.2	(kopia robocza)
@@ -82,6 +82,9 @@
 .Fa s
 argument
 is not a valid descriptor.
+.It Bq Er EINVAL
+The socket is already bound to an address, and the protocol does not support
+binding to a new address; or the socket has been shut down.
 .It Bq Er ENOTSOCK
 The
 .Fa s