MEDIUM: acl: fix the initialization order of the ACL expression

MEDIUM: acl: fix the initialization order of the ACL expression

The ACL expression parser recently became a huge mess like a
spaghetti plate. The keyword is looked up at the beginning, then
sample fetches are processed, then an expression is initialized,
then arguments and converters are parsed but only if the keyword
was an ACL one, etc... Lots of "if" and redundant variables
everywhere making it hard to read and follow.

Let's move the args/conv parsing just after the keyword lookup.
At least now it's consistent that when we leave this if/else
statement, we have a sample expression initialized and full
parsed wherever the elements came from.