clone: send server options when using protocol v2

clone: send server options when using protocol v2

Commit 5e3548ef16 ("fetch: send server options when using protocol v2",
2018-04-24) taught "fetch" the ability to send server options when using
protocol v2, but not "clone". This ability is triggered by "-o" or
"--server-option".

Teach "clone" the same ability, except that because "clone" already
has "-o" for another parameter, teach "clone" only to receive
"--server-option".

Explain in the documentation, both for clone and for fetch, that server
handling of server options are server-specific. This is similar to
receive-pack's handling of push options - currently, they are just sent
to hooks to interpret as they see fit.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>