nat: T6371: fix op mode display of configured ports when comma separated list of ports/ranges exists

Before: Issuing the op mode command "show nat source rules" will throw an
exception if the user has configured NAT rules using a list of ports as a
comma-separated list (e.g. '!22,telnet,http,123,1001-1005'). Also there was
no handling for the "!" rule and so '!53' would display as '53'.

With this PR: Introduced iteration to capture all configured ports and append
to the appropriate string for display to the user as well as handling of '!' if
present in user's configuration.