s_client accepts host/port as positional argument.

This allows the user to provide the target host and optional port to
openssl s_client as an optional positional argument, rather than as the
argument to the -connect flag. This rationalises the user experience of
s_client: given that the only logical purpose of s_client is to connect
to a host, it is difficult to understand why there is an (effectively
mandatory) command option to pass to make that happen.

This patch forbids providing *both* -connect and the positional
argument, because it would likely be too difficult to reconcile.
Otherwise, using the positional argument behaves exactly the same as
using -connect does.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1171)