Fix potential infinite loop when adding new agent using file input

When adding a new agent using the -f option provided by manage_agents
there is a possibility that it loops infinitely if you have used up all
of the potential IDs. It will say that the ID needs to be unique since
the last ID checked is already in use. This commit adds a new message
stating the problem and prevents the infinite loop. It also increases
the amount of IDs manage_agents will look at when adding new agents both
in the interactive mode and when using the -f option.