MINOR: acl: add acl_cond_find_positive_use() to find positive terms by usage

MINOR: acl: add acl_cond_find_positive_use() to find positive terms by usage

This function looks for a term of a condition which makes the condition
true when it matches (a non-negated term after "if", or a negated one
after "unless") and which involves a sample fetch having certain
SMP_USE_* bits. It returns the first matching ACL and keyword, in the
same way as acl_cond_kw_conflicts() does. The combination of terms is
not considered, a single positive term is enough.

It will be used to detect deny-lists built on aliasable contents.