Key management

[lee-lindley]

Resolves #16
I have not tested extensively with other user accounts (only 1 from anonymous sql). An implementer can allow each login user to manage their own keys/known hosts, or else can create a 2nd package for them to use that will centralize everything under the username of that package owner (can be same as owner of as_sftp).
The fine grained access control is optional. The key management works without it, but anyone who can select from the table will be able to obtain the private keys.

I did not add anything to the README.md or usage.md. My style is very different from yours and I don't want to presume.
I would be happy to create a document similar to others you can see it my github repository, but I'm not sure you want to maintain something like that.

I won't object if you reject the pull request but use my work to do it your way. I had fun and that was enough.

HTH.

[lee-lindley]

I have pictures and umlet .uxf source files for them. Can add a directory to the pull request branch if you want them.
I know why I provided the as_sftp_shared* code, but it is hard to explain without the pictures. Without it the implementation requires each calling user or package owner to maintain their own keys (which is fine for most people I suspect).