RunCmd: Expose "postfork" callback

The “_postfork_fn” parameter was only used for tests until now. To
implement a good locking scheme, remote commands must also make use of
this callback to release a lock when the command was successfully
started (but did not yet finish).

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>