Add flag to set database path

[romanbuk52]

I have added a flag to configure the path to the database, I think it's not okay to save configs in a root.

[ngoduykhanh]

Thank you for your PR. What is the difference between saving the db in binary's root folder (./db ) and /etc/wireguard/db? I think having a custom path for it makes sense. However, can you instead use the existing path (./db) as the default value?

[romanbuk52]

Okay, will change the default value and I will let you know