With this commit, the server will query am external group list provider to know what are the alt groups (or secondary groups)
for a user that does a request to the server. The server will then trust what getgroup() tells and won't trust altgroups
provided in AUTH_SYS creds.
Interesting consequence is a better centralization of groups management. At the same time, this breaks the 16 altgroup array size
in the AUTH_SYS credentials which lead to EPERM. If the server uses the feature introduced by this commit, user may belong to
more than 16 groups : they all will be considered. None will be forgotten as it occurred with AUTH_SYS.

This commit reuse deprecated EXPORT_OPTION_USE_PNFS flag
Result of getgroup() are kept in a cache (in fact a AVL tree). Information stored in cache expires and are regularily renewed. Expiration is tunable.

This commit add two configuration items:
	- a boolean "Do_Not_Trust_AuthSys_Grp" (default = FALSE), which is a per-export option
	- an integer "Extended_Groups_Expiration" (default = 30), which is a global parameter in NFS_Core_Param block.