Fix an error in setfacl(1) that manifested like this:

Fix an error in setfacl(1) that manifested like this:

# setfacl -m u:trasz:rwx x
# setfacl -m u:root:rwx x
# getfacl x
# file: x
# owner: root
# group: wheel
user::rw-
user:root:rwx
user:trasz:rwx
group::r--
mask::rwx
other::r--
# setfacl -m u:root:rwx x
setfacl: x: acl_calc_mask() failed: Invalid argument
setfacl: x: failed to set ACL mask

For NFSv4 ACLs, this sort of situation would result in duplicated
entries.

MFC after:	1 month