[rpm] oneshot is actual only for some systems

Adding --with(out)-oneshot option to allow to build statefs on systems
without oneshot

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[tests] fixed unit tests

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
removing dead code

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
link test: check for duplicate instances

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[lib] FunctionSource and BasicNamespace

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
do not ignore setuid/gid errors

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>