NewAllocator return a PortAllocator.

ErrNoFreePort is returned when trying to reserve a port but all the the port of the range have been already reserved.

Allocator implements the PortAllocator interface.

PortRange hold the beginging and end of a range of port a PortAllocator can reserve.

PortAllocator is the interface that defines the behavior to reserve a port in a specific network namespace.