Add new gateway allow name duplication
When adding a new gateway, it doesn't check name collision with disabled interfaces and in the end you have 2 gateways with the same name.
Steps to reproduce:
1. Add a new gateway called 'foo' to interface INT1
2. Disable interface INT1
3. Add a new gateway to another interface called 'foo'
Make return_gateways_array() return all disabled gateways when $disabled is true, it should fix #3291