Initialise all struct FS members

The struct FS constructor initialised every member *except* filesystem
and busy.  Then in *most* cases after declaring struct FS, assignments
followed like this:
    FS fs;
    fs.filesystem = FS_BTRFS;
    fs.busy       = FS::GPARTED;
But member busy wasn't always initialised.

Add initialisation of members filesystem and busy to the struct FS
constructor.  Specify optional parameter to the constructor to set the
filesystem member, or when left off filesystem is initialised to
FS_UNKNOWN.