Support for POSIX ACLs

* configure.ac: Check whether ACLs are available on the
host system.
* gnulib.modules: Add acl
* src/create.c (start_header): Store ACLs when creating
a POSIX archive.
(dump_file0): Handle ACLs.
* src/extract.c (delayed_set_stat) <acls_a_ptr, acls_a_len>
<acls_d_ptr, acls_d_len>: New members.
(delayed_link): Likewise.
(set_stat,delay_set_stat)
(apply_nonancestor_delayed_set_stat): Set ACLs.
* src/tar.c: New options: "--acls", "--no-acls"
(tar_stat_destroy): Free acls_a_ptr and acls_d_ptr fields.
* src/tar.h (tar_stat_info) <acls_a_ptr, acls_a_len>
<acls_d_ptr, acls_d_len>: New members.
* src/xattrs.c (xattrs_acls_get, xattrs_acls_set): New functions.
* src/xheader.c: Support new keywors: "SCHILY.acl.access" and
"SCHILY.acl.default".
* tests/Makefile.am: Add new tests.
* tests/testsuite.at: Likewise.
* tests/acls01.at: New test.
* tests/acls02.at: New test.