Update txeh.go

[docooler]

fix ipv4 and ipv6 using localhost at same time

[cjimti]

@docooler please provide an explanation on this and an example use case I can test. This could be a breaking change for some people so I want to better understand what you are trying do.

[docooler]

we have below /etc/hosts
#localhost name resolution is handled within DNS itself.
127.0.0.1 localhost
::1 localhost

using cli txeh add 127.0.0.1 localhost.localdomain add a new host to ipv4 localhost.
it will change to below
#localhost name resolution is handled within DNS itself.
127.0.0.1 localhost localhost.localdomain
::1 localhost

using cli txeh add ::1 localhost.localdomain add a new host to ipv6 localhost.
it will change to below
#localhost name resolution is handled within DNS itself.
127.0.0.1 localhost
::1 localhost localhost.localdomain

But I expect it change to
#localhost name resolution is handled within DNS itself.
127.0.0.1 localhost localhost.localdomain
::1 localhost localhost.localdomain

so I commit this PR.
thx