Change funptr's [release_runtime_lock] argument to [runtime_lock].

The [funptr] view can be used in either direction.  When [funptr] is
used to call C from OCaml, [runtime_lock] determines whether the lock
is released.  When [funptr] is used to call OCaml from C,
[runtime_lock] determines whether the lock is acquired.
Optionally acquire the runtime lock in the callback handler.
A test for the [runtime_lock] argument to [funptr].
Merge pull request #171 from yallop/funptr-acquire-lock

Support for acquiring the runtime lock in callbacks