BUG/MINOR: server: duplicate server alt_proto in srv_settings_cpy()

Server <alt_proto> may be set to select an alternative protocol to the
default one depending on the server address type. Currently, the main
usage is for MPTCP on the backend side.

This field was not duplicated in srv_settings_cpy(). This has no impact
with default-server as such entry do not have any address configured.
However, this causes an issue with server-template, as duplicated
entries will remain with an unset <alt_proto> field. This results in an
incorrect TCP protocol selected on connect instead of MPTCP.

Fix this by ensuring that <alt_proto> is copied in srv_settings_cpy().
This is only performed for server-templates, as with other settings
relative to the server address.

This must be backported up to 3.2.