SetEtcHostsEntry: maintain existing ordering

Currently RemoveEtcHostsEntry keeps the ordering, but SetEtcHostsEntry
not, as it will always write the new entry at the end of file. I
personally dislike this as it "uglifies" my custom host files, so this
patch makes it update the record instead in-place so to say instead of
moving it.

The patch also simplifies the construction of the new line (we were
doing duplicate work for no gain).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>