Don't set server usage for client certificates

[travisgroth]

Closes #273

[gaby]

@FiloSottile Any plans to merge this?

[gaby]

@travisgroth Can you rebase this pr ?

[moparisthebest]

letsencrypt certs have both server and client set:

X509v3 Extended Key Usage: 
                TLS Web Server Authentication, TLS Web Client Authentication

Which is required for protocols like XMPP

So currently I'm using -client to create certificates like letsencrypt makes to test XMPP things, it'd be a shame if that was removed. Probably what should happen is without -client it should add both key usages and then -client could take away only TLS Web Server Authentication ?

[gaby]

@moparisthebest Maybe there should be different modes for this flags. Like default should be (server + client). If the -client then only client, if -server then only server?