Handle failures from seteuid(0) in su_quotactl() before calling
quotactl(). This fixes the warn_unused_result compiler warning and
returns an error to the quota caller if privilege elevation fails.
SYSCON reads volume/disk restrictions before opening the edit dialog.
For volumes that do not map to a quota-capable block device, such as
HOME on /home, the old code returned an error during the read path.
This made SYSCON abort with error -30312 before the user could even see
the restriction state.
Treat read requests on volumes without a quota backend as unrestricted:
return success with an unlimited quota and zero usage. Removing a
restriction on such a volume is also treated as a successful no-op.
Setting a non-zero restriction still fails, so admin tools do not report
a quota as enforced when no Linux quota backend exists.
