ofproto-dpif: Add boottime support field.

ofproto-dpif: Add boottime support field.

When changing support fields, it may be unsafe to set support level
beyond what datapath can support.

This patch introduce the notion of boot time support and
runtime support fields. Boot time support are set only
once during ofproto start up phase, and not changed during
runtime. The runtime support fields are the same as boot time
support fields at the startup time, but can be changed via
the 'ovs-appctl' command.  However, each change will
be checked against the corresponding boot time support field. Only
feature reduction from the boot time support is allowed.

Signed-off-by: Andy Zhou <azhou@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>