Virtual ID mapping functionality.

Support for remapping user and group IDs e.g. for ID namespace
virtualization purposes.
The feature is enabled by the VID_MAPPING CMake option and defining
a "Script" parameter in the "VID_Mapping" configuration block.
The script shall accept arguments in the form:

script_name XIDTOID|IDTOXID UID|GID <ID>

and print out the answer in the form:

UID|GID: <ID>

The mappings are cached and their lifetime can be controlled by
setting the "Expiration_Time" parameter.

Signed-Off-By: Daniel Gan-Levi <danielg@il.ibm.com>
Virtual ID mapping - remap anonymous IDs too.

Anonymous IDs, if configured, should be considered external IDs
(not virtual), in terms of virtual ID mapping.