Fix config bug related to port setting

Fix config bug related to port setting

There are three levels there for the port value, with increasing
priority:

1. The default ports, defined by 'port' and 'tls-port' config options.
2. The primaries-level default port: primaries port <number>  { ... };
3. The primaries element-level port: primaries { <address> port
   <number>; ... };"

In 'named_config_getipandkeylist()', the 'def_port' and 'def_tlsport'
variables are extracted from level 1. The 'port' variable is extracted
from the level 2. Currently if that is unset, it defaults to the
default port ('def_port' or 'def_tlsport' depending on the transport
used), but overrides the level 2 port setting for the next primaries in
the list.

Update the code such that we inherit the port only if the level 3 port
is not set, and inherit from the default ports if the level 2 port is
also not set.