History API heads up

[Alex Kiernan]

I'm going to start landing this in the next few days, whilst the
changes look widespread, much of it is just consolidating the history
access stuff to go through the library.

Theres a few wrinkles need tidying:

- dbz.c, dbz.h should probably move into the libhist/hisv6

- makedbz needs changing to make calls into the hisv6 library (but
probably back door methods) and moving out of expire/

- tagged hash history needs hauling out into a separate implementation

- `#ifdef CANT_DO_THIS' in expire.c needs resolving (the -i and -x
options)

- there are doubtless some bugs/misfeatures which I've missed (not
least as its so long since I started this I'll have forgotten bits)

- there's some cleanup in line with Russ' recent changes

- there are some user visible changes, principally makehistory now
generates the dbz files (again), hence supports the -s option,
grephistory loses the -h (hash a messageid) and -t (print offset into
history file) options, also inn.conf acquires a hismethod parameter
(which can only be hisv6 right now).

AFAIK its fully backward compatible - I've been swapping between 2.3.2
and 2.4-current regularly without rebuilding stuff.

-- 
Alex Kiernan, Principal Engineer, Development, Thus PLC