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.

(cherry picked from commit b7595ee9d328778105c70e3d4399ac45f555b304)