acl: add heuristic to detect acl format

there are two ways to define acl:

USER:7:rlwfx:o:ALLOW:FFFF
and
USER:7:i+rlwfx:o:FFFF

make ACE#parse() smart enouth to detect that.

Ticket: #8424
Acked-by: Karsten Schwank
Target: master, 2.10
Require-book: no
Require-notes: no