POSIXUsers: set managed uid/gid range

Added options to set a range (whitelist or blacklist) of managed
uids/gids so that accounts in LDAP (e.g.) do not get flagged as
"extra" entries.

Request: http://article.gmane.org/gmane.comp.sysutils.bcfg2.devel/4629
GroupPatterns: improved PackedDigitRange and tests