excessive default users in mk/defaults/mk.conf

[Greg Troxel <gdt%lexort.com@localhost>]

For reasons that are unclear to me, we have many default users in
mk/defaults/mk.conf and some in packages (sometimes via a
Makefile.common).  Some of the defaults ones are used in multiple
coordinated packages that don't have shared builds, but only apache and
games have a justification in the config file.

Variables like this seem better suited for a Makefile.common or a
Makefile.users in most cases.  I suggest that some to many of these
cases are bugs and that demoting them to category/package/Makefile or
Makefile.users would be better.  I may do this for jabberd, as something
that I feel slightly responsible for, after checking more carefully.

mk.conf contains:

AMANDA_USER?=   amandabackup
APACHE_USER?=   www
BINCIMAP_USER?= bincimap
BIND_USER?=             named
CACTI_USER?=            cacti
CANNA_USER?=    daemon
CLAMAV_USER?=   clamav
CUPS_USER?=     lp
CYRUS_USER?=    cyrus
DBUS_USER?=     dbus
DEFANG_USER?=   defang
DQCACHE_USER?=  dqcache
EXIM_USER?=     mail
FREEWNN_USER?=  wnn
GAMES_USER?=    games
HOWL_USER?= howl
INN_USER?=      news
JABBERD_USER?= jabberd
LEAFNODE_USER?= news
MEDIATOMB_USER?= mediatomb
MIREDO_USER?=           miredo
MLDONKEY_USER?= mldonkey
MONOTONE_USER?= monotone
NAGIOS_USER?=   nagios
NSD_USER?=              nsd
NULLMAILER_USER?=       nullmail
OPENSSH_USER?=          sshd
QPOPPER_USER?= pop
SCREWS_USER?=   screws
SHAIRPORT_USER?= shairport
SNIPROXY_USER?=         sniproxy
TINYDYN_USER?=  tinydyn
UNBOUND_USER?=          unbound
UUCP_USER?=     uucp