CLEANUP: acl: cleanup some of the redundancy and spaghetti after last fix

CLEANUP: acl: cleanup some of the redundancy and spaghetti after last fix

This code aims at clearing up the ACL parsing code a bit to make it
more obvious what happens in the case of an ACL keyword and what happens
in the case of a sample expression. Variables prev_type and cur_type were
merged, and ACL keyword processing functions are checked only once.

This patch could be backported into 1.5 after the previous patch, in order
to keep the code more maintainable.