Adding Pid to new_data message

Not having the PO Box's pid in the new_data message means that the
current PO Box implementation can only be used safely with a single box
per process in notify mode. Active mode already contains the pid, and so
is safe to use with multiple PO Boxes.

This commit adds the pid of the po box to the new_data message so that a
process owning multiple PO Boxes can know which one sent the mail
notification so it can safely be polled.
Using Size when dropping items

POBox tracks the size of all the entries it has for quick matching to
know when a buffer is full or not. However, it does *not* make use of
that data when dropping more than 1 element (either in drop or
push_drop) and instead recalculates the buffer size dynamically (O(n)).

This behaviour can make it so busy buffers that need to drop data
continuously because they're full end up doing a lot more work than
nearly-empty buffers, which is counter-productive (they should all be
doing nearly no work at all).

This patch makes it so that the size is explicitly passed to the drop
functions of the module, bringing that length check to a no-cost
operation.