-spersistent changed the requirements a fair bit, and amongst other
things introduced a ban timestamp as a unique identifier of a ban,
and the need to save and reinstate a ban.

The way this was implemented somewhat less than elegant, but we had
little choice due to the way regexps work, and amongst other horrors
it involved too much parsing and quoting of bans as text strings.

Since then we have switched to PCRE, which compiles regexps to
a bytestream which can be saved offline, and this paved the way
for this rewrite.

Instead of a datastructure with a list of tests to be carried out,
build a byte-string which encodes the same information, along with
additional information (the uncompiled regexp) to allow the ban to
be reconstructed for display in ban.list.

Include the ban timestamp as the first 8 bytes of the ban specification,
to eliminate obj->ban_t, saving 8 bytes per object.

Additional polishing to be expected.

private to cache_ban.c, so inline it there.

apply to individual pools.

precisely.

It does not make sense to specify less that 1M of storage, so error
out if a user specifies -smalloc,512 for instance.  They probably
meant -smalloc,512M, but require them to be explicit.

Adjust C00027 test case to match

reworked from a commit to 2.1 by Bjørn Ruberg.