Count *ring buffer blocks*, not *packets* to be filtered in userland.

We count the number of free blocks in the ring buffer, and calculate the
number of blocks in the ring buffer containing packets.  With TPACKET_V1
and TPACKET_V2, there's one packet per block, but, with TPACKET_V3,
there can be more than one packet per block, so we need to count blocks,
not packets, to decide when we no longer need to filter packets in
userland.

Don't overload the "filtering in kernel" flag to hold the count of
blocks; instead, have a "filter in userland" flag, indicating that we
have to filter packets in userland (whether that's because the filter
wasn't accepted by the kernel or because we need to deal with packets
that passed an earlier kernel filter but wouldn't have passed the new
kernel filter) and a count of blocks to filter in userland.

After counting the number of free blocks, if we find any, decrement that
count by 1, just in case some packet that had passed the old filter got
added to the ring after we counted the free blocks.  (For TPACKET_V1 and
TPACKET_V2, that would mark the block as no longer free, so the window
in which that could happen is narrower than for TPACKET_V3, where the
block might not be full and the timeout might not have expired, so it
could take some time before that block is handed to userland.)

Should fix GitHub issue #331.

(While we're at it, use the term "block" consistently for a fixed-length
item in the ring.)
Formatting tweak.
Add items for 1.5.1 and 1.5.2.
Tag some changes with a bug identifier.
Add post-1.5.2 bug fixes.

Also reformat some items, and give more details on one of the new-in-1.5
features.