CLEANUP: stktable: replace nopurge attribute with flag

CLEANUP: stktable: replace nopurge attribute with flag

Thanks to previous commit stktable struct now have a "flags" struct member

Let's take this opportunity to remove the isolated "nopurge" attribute in
stktable struct and rely on a flag named STK_FL_NOPURGE instead.

This helps to better organize stktable struct members.