libsmack: parse whole access type string, not only first 5 bytes.

Previous version of this function parsed only first ACC_LEN (5)
characters of access_type. Now the whole string will be read.

This will prevent silent ignoring of access type characters in cases
like "-rwxat" or "rrwxat".
libsmack: add support for modification rules.

A new API function smack_accesses_add_modify() is provided for adding
modification rule to a set. Rule specifies permissions to grant and
permissions to revoke. If a rule for given subject and object already
existed, unmentioned permissions will be left unchanged.
Also smack_access_save() and smack_access_add_from_file() are modified
to support reading and writing such rules from/to a file.
Merge pull request #11 from rafal-krypa/samsung

Samsung