MINOR: pattern: new sflag PAT_SF_REGFREE indicates regex_free() is needed

MINOR: pattern: new sflag PAT_SF_REGFREE indicates regex_free() is needed

Currently we have no way to know how to delete/prune a pattern in a
generic way. A pattern doesn't contain its own type so we don't know
what function to call. Tree nodes are roughly OK but not lists where
regex are possible. Let's add one new bit for sflags at index time to
indicate that regex_free() will be needed upon deletion. It's not used
for now.