Knownhosts tilde aware path expansion - fixes #1107

[memetb]

This directly addresses issue #1107 but is also a small refactor effort to move some utility code into the util package with associated unit tests.

The specific changes are to move the file path expansion code to a dedicated utility function in the util package and add an associated unit test.

As a note, issue #1107 is legacy to v0.8.x and existed before here.

if doVerify {
		cb, err := knownhosts.New(os.ExpandEnv(knownHostsPath)) // <- this does not expand tilde
		if err != nil {
			return nil, fmt.Errorf("failed to read ssh known hosts: %w", err)
		}
		hostKeyCallback = cb
	}

[dmacvicar]

I won't call this monkey patching. It is just a drop-in replacement function.

The part I think we can improve is the naming. We are replacing os.ExpandEnv, which expands generic string, for something that is specialized into paths, so why not just ExpandPath ?

Even its argument is called path.