vdev_id: implement slot numbering by port id

With HPE hardware and hpsa-driven SAS adapters, only a single phy is
reported, but no individual per-port phys (ie. no phy* entry below
port_dir), which breaks topology detection in the current sas_handler
code. Instead, slot information can be derived directly from the port
number. This change implements a new slot keyword "port" similar to
"id" and "lun", and assumes a default phy/port of 0 if no individual
phy entry can be found. It allows to use the "sas_direct" topology with
current HPE Dxxxx and Apollo 45xx JBODs.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Daniel Kobras <d.kobras@science-computing.de>
Closes #6484