MINOR: acl: Extract the pattern parsing and indexation from the "acl_read_patterns_fr...

MINOR: acl: Extract the pattern parsing and indexation from the "acl_read_patterns_from_file()" function

With this split, the pattern indexation can apply to any source. The map
feature needs this functionality because the map cannot be loaded with the
same file format as the ones supported by acl_read_patterns_from_file().

The code was only moved to its own function, no functional changes were made.