Add lockfile.GetLockFile and lockfile.GetROLockFile (with upper-case
F, as opposed to existing methods with lower-case f) which return a
*LockFile struct, instead of the Locker interface.
This follows the general Go design heuristic: "return structs, accept
interfaces"; more importantly, it allows us to add more methods to the lock
object without breaking the Go API by extending a pre-existing interface.
Should not change behavior.
Signed-off-by: Miloslav Trmač <mitr@redhat.com>