file_data: cleanup common accessor functions (#641)
file_data: cleanup common accessor functions (#641)

fd.file.tailPointer() -> fd.rootBlockPointer()

Add childIPtr() accessor to parentBlockAndChildIndex.

Suggested by jzila.

Issue: #621
keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/kbfs,keybase/kbfs,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client