When adding a host domain, optimize the hostentry

[credmp]

When adding example.com when a subdomain demo.example.com already exists, make the top level entry the name and move the subdomain to aliasses.

./hed --file test.txt show
Reading file test.txt
10.10.10.10 demo.example.com    
 ~/Downloads $ ./hed --file test.txt add example.com 
Reading file test.txt
Error: 
   0: Could not add host, no parent domain to resolve it.

Location:
   src/main.rs:177

Backtrace omitted.
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