(acl_entries): Add fallback implementation for POSIX ACL

(acl_entries): Add fallback implementation for POSIX ACL
systems other than Linux.
(chmod_or_fchmod): New function: use fchmod when possible,
and chmod otherwise.
(file_has_acl): Add a POSIX ACL implementation, with a
Linux-specific subcase.
(copy_acl): Add: copy an acl and S_ISUID, S_ISGID, and
S_ISVTX from one file to another.  Fall back to fchmod/chmod when
acls are unsupported.
(set_acl): Add: set a file's acl and S_ISUID, S_ISGID, and
S_ISVTX to a defined value.  Fall back to fchmod/chmod when acls
are unsupported.