Allow Local authentication for Negotiate

Allow Local authentication for Negotiate

PeerCred is also possible if address family is AF_LOCAL. This will allow
the CGI programs to generate the authorization from the local
certificates based on PID also when Negotiate is used for local
connections:

Client                       CGI
Browser <- Remote conn -> admin.cgi <--- Localhost conn --->  Scheduler
  |                           |                                    |
  + --- HTTP/POST /admin/ --> |                                    |
  |                           + --- CUPS-Get-Devices ------------> |
  |                           |                                    |
  |                           | <-- 401 Unauthorized --------------+
  |                           |     WWW-Authenticate:              |
  |                           |       Negotiate, (PeerCred,) Local |
  |                           |                                    |
  | <-- 401 Unauthorized -----+                                    |
  |     WWW-Authenticate:     |                                    |
  |       Negotiate           |                                    |
  |                           |                                    |
  | --- HTTP/POST /admin/ --> |                                    |
  |     Authorization:        + --- IPP CUPS-GetDevices ---------> |
  |       Negotiate           |     Authorization: Local <cert>    |
  |                           |                                    |

Fixes: #5596
Signed-off-by: Samuel Cabrero <scabrero@suse.de>