Support multiple hostnames

[rtpt-erikgeiser]

This PR adds optional support for multiple hostnames. It can be enabled during compilation via -ldflags "-X main.enableMultipleHostnames=true" or during runtime with an environment variable (export DB_NMAP_ENABLE_MULTIPLE_HOSTNAMES=1). If a host has multiple hostnames they will be joined with a colon like this: hostname1, hostname2

The PR also adds a missing error check.