datapaths: Improve NO_RECV port config flags

The previous implementation of OFPPC_NO_RECV didn't honor the STP-related
semantics laid out in the spec, and OFPPC_NO_RECV_STP was not implemented
at all.  This commit attempts to remedy that.
datapaths: Refactor/rewrite port_mod handling

This is an attempt to rewrite how port config flags are handled for
port_mods.  The goals are to reduce duplicated code and to improve the
ability for subclasses to override behavior.
datapaths: Improve port config

1. Set STP-disable flag by default
2. Don't allow enabling of STP
3. All other port config flags are handled
flow_table: Remove SwitchFlowTable subclass

All this subclass did was process flow_mod entries.  This functionality
has been integrated into SoftwareSwitchBase.  So as not to have to
maintain two versions of similar code, we remove it here.