Bug#4062 - Support PID variable in HiddenStores filename.
Update NEWS and mod_xfer docs with info about the HiddenStores %P variable.

Started working on new RELEASE_NOTES for this release.
Merge pull request #9 from proftpd/hiddenstores-pid-variable-bug4062

Bug#4062 - Support PID variable in HiddenStores filename.