Set salt for 'rekey' command in nudb tool

[MarkusTeufelberger]

Currently ever key file that gets created is different due to the different salt that gets chosen. It would be nice to be able to optionally specify the salt.

[vinniefalco]

pull request?

[MarkusTeufelberger]

Do you accept them? There are some mergeable ones in the pipeline for years by now...

[MarkusTeufelberger]

Well, no answer is also an answer I guess.

[vinniefalco]

We can do it