[doc] inout documentation, package is renamed

Added InOut loader documentation, package is renamed from
statefs-provider-doc to statefs-doc

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[rpm] build requires doxygen

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

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[inout] implementation of inout loader

Implementation of inout/mirror loader. This loader uses statefs
configuration files as a source of information to create input and
output namespaces with data written to files in the input namespace just
appeared in correponding files in the output namespace

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