Add ACL scheme for checking that user belongs to nss group
* Add documentation for *unxgrp* ACL scheme
* Add *unxgrp* in supported ACL scheme in usage message
* Add test suite for acl-unxgrp
Remove useless debug message from fake `getgrnam_r` source code.
Remove test only directive that shouldn't have been pushed on this PR.
New implementation of `unxgrp` ACL scheme.

* As suggested by @rra, the principal is now converted to local
user name with `krb5_aname_to_localname`.
* Fix LICENSE issue by removing CeCILL headers and falling back
to global remctl license.
* Improve test suite to check for proper error handling.

Implementation remarks:

* As `krb5_aname_to_localname` requires principal to be from a known
realm to work, test suite now had to be run with a kerberos configuration
file that defines at least the realm `EXAMPLE.ORG`. I'd liked to use
function `kerberos_generate_conf()` defined in `tests/tap/kerberos.c`
but I wasn't able to find the script `tests/data/generate-krb5-conf`.
My solution was to use the sample configuration file provided with the java
bindings `java/k5.conf`.

If no realm `EXAMPLE.ORG` could be found in kerberos configuration file,
tests will fail like this:

```bash
$ ./tests/runtests -o ./tests/server/acl-unxgrp
1..14
ok 1 - ... with empty group
not ok 2 - ... with user within group
ok 3 - ... with user not in group
ok 4 - ... with principal with instances but main user in group
ok 5 - ... with long_principal very very long
ok 6 - ... match error message with principal too long
ok 7 - ... with user from not supported REALM
ok 8 - ... with getgrnam_r failing
ok 9 - ... with getgrnam_r error handling
ok 10 - ... with denied user in group
ok 11 - ... with user not in denied group but not allowed
not ok 12 - ... with user within group plus a deny pragma
ok 13 - ... with user in denied group plus a allow group pragma
ok 14 - ... with user neither in allowed or denied group
```
but with the sample configuration file shipped with the java bindings:

```bash
$ KRB5_CONFIG=../java/k5.conf ./tests/runtests -o ./tests/server/acl-unxgrp
1..14
ok 1 - ... with empty group
ok 2 - ... with user within group
ok 3 - ... with user not in group
ok 4 - ... with principal with instances but main user in group
ok 5 - ... with long_principal very very long
ok 6 - ... match error message with principal too long
ok 7 - ... with user from not supported REALM
ok 8 - ... with getgrnam_r failing
ok 9 - ... with getgrnam_r error handling
ok 10 - ... with denied user in group
ok 11 - ... with user not in denied group but not allowed
ok 12 - ... with user within group plus a deny pragma
ok 13 - ... with user in denied group plus a allow group pragma
ok 14 - ... with user neither in allowed or denied group
```