Add a FindQuota.cmake module that checks for the Linux quota headers and
symbols required by the MARS NWE quota backend. Prefer modern
<sys/quota.h> with quotactl(), and keep <linux/quota.h> as fallback for
older systems.
Enable quota support by default, but only compile it in when the required
headers and quota symbols are available. Also move the QTAINSYS define to
the detected sys/quota.h path so modern Linux builds use the correct
header automatically.
