Advertizing information about unprivileged user namespaces in glidein classad

[namrathaurs]

Fixes #371

As identified in the issue description, the cvmfs_helper_funcs.sh contains a utility has_unpriv_userns that can be used to fetch information whether unprivileged user namespaces are (1) supported and (2) enabled on the worker node. This utility internally uses the variables GWMS_IS_UNPRIV_USERNS_SUPPORTED and GWMS_IS_UNPRIV_USERNS_ENABLED. The utility returns one of four values:

• enabled: unprivileged user namespaces is supported and enabled on the worker node
• disabled: unprivileged user namespaces is supported but not enabled on the worker node
• unavailable: unprivileged user namespaces is neither supported nor enabled on the worker node
• error: unprivileged user namespaces is not supported but enabled on the worker node

It is this value that gets advertized to the glidein classad and can be accessed using HAS_UNPRIVILEGED_USER_NAMESPACES attribute.

As an example, if the worker node supports unprivileged user namespaces and has unprivileged user namespaces enabled, the glidein classad on the client should show:

[testuser@gwms-frontend ~]$ condor_status -l slot1@glidein_1649634_917375096@fermicloudxxx.fnal.gov | grep -i "namespaces"
HAS_UNPRIVILEGED_USER_NAMESPACES = "enabled"
HasUserNamespaces = true

Additionally, the same variable is also exported to the user job environment (with the same name HAS_UNPRIVILEGED_USER_NAMESPACES).