Made a few encryption things simpler:

* Only one strict/lax setting, in [encryption], rather than separate
  settings in [properties] and [sshkeys]
* No longer necessary to enable encryption on each Properties file
read default Path metadata from config file properly
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
docs: added docs for POSIXUsers uid/gid ranges
Properties: fixed lax/strict decryption setting with no crypto libs installed