completion**users ignored-patterns more robust

Calculates ignored users at runtime by looking at passwd
database and stripping all users with UID < 500. This should
work on most unices except perhaps Solaris, which supposedly
starts normal users at UID=100.

By my timings, this makes it only some 0.010s slower.

modules/completion/init.zsh

@@ -99,14 +99,8 @@ zstyle -e ':completion:*:hosts' hosts 'reply=(
 )'
 
 # Don't complete uninteresting users...
-zstyle ':completion:*:*:*:users' ignored-patterns \
-  adm amanda apache avahi beaglidx bin cacti canna clamav daemon \
-  dbus distcache dovecot fax ftp games gdm gkrellmd gopher \
-  hacluster haldaemon halt hsqldb ident junkbust ldap lp mail \
-  mailman mailnull mldonkey mysql nagios \
-  named netdump news nfsnobody nobody nscd ntp nut nx openvpn \
-  operator pcap postfix postgres privoxy pulse pvm quagga radvd \
-  rpc rpcuser rpm shutdown squid sshd sync uucp vcsa xfs '_*'
+zstyle ':completion:*:*:*:users' ignored-patterns nobody '_*' \
+  $(awk -F':' '$3 < 500 && $1 != "root" {printf "%s ", $1}' /etc/passwd)
 
 # ... unless we really want to.
 zstyle '*' single-ignored show