Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>

Previously the sma lock was used as for communication with
responder. However it would cause a deadlock in case of
re-checking memcache after acquiring the lock and before communication with
responder..

Required by:
https://fedorahosted.org/sssd/ticket/2581

Reviewed-by: Michal Židek <mzidek@redhat.com>

Although errno was cleared in function sss_nss_make_request
some sss glic functions set errno with value of output argument errnop.

Reproducer:
* sssd compiled with enabled option sss-default-nss-plugin
* sss is the last value in group (/etc/nsswitch.conf)
* sssd-client is installed but sssd is stopped.

C-program:
  #include <stdio.h>
  #include <stdlib.h>
  #include <errno.h>
  #include <grp.h>

  int main(int argc, char *argv[])
  {
      struct group *p_group;

      setgrent();
      while (1) {
          errno = 0;  /* initialize for  getgrent() */
          p_group = getgrent();
          if (p_group == NULL) {
              if (errno == 0) {
                      break;   /* end of groups */
              } else {
                  perror("getgrent");
                  printf("getgrent error %d \n", errno);
                  endgrent();
                  exit(-2);
              }
          }
          printf("getgrent() OK group(%d) = %s \n",
                 p_group->gr_gid, p_group->gr_name);
      }

      exit(0);
  }

Resolves:
https://fedorahosted.org/sssd/ticket/2619

Reviewed-by: Pavel Reichl <preichl@redhat.com>

The PAC responder by default allows only connections from the root user.
This patch opens the socket to the PAC responder before the krb5_child
drops privileges so the connection seemingly comes from root.

https://fedorahosted.org/sssd/ticket/2559

Reviewed-by: Sumit Bose <sbose@redhat.com>

glibc is inconsistent with how it treats and returns NSS_STATUS_UNAVAIL.

The sss nss plugin is present in nsswitch by default on some platforms
due to glibc caching and problem with long living applications (e.g. GNOME).
But sssd needn't be configuread and it cause problems in some programs.
In this situation, the SSSD nss plugin should behave as if it was functioning
but had no data even thought sssd is not running. The errors have to be passed
from nss plugin up to the user with minimal moidiffication.

Thanks to Stephen Gallagher for initial patch.

Resolves:
https://fedorahosted.org/sssd/ticket/2439

Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>

We had in source code following pattern #elif HAVE_<name>
It worked because undefined identifier(in some cases) was evaluated to 0.
But we do not care about value of HAVE_SOMETHING. We just need to know
whether identifier was defined.

There is not equivalent to #ifdef (short for of #if definded)
We need to use long form: #elif defined HAVE_<name>

It causes also compiler warning with enabled compiler flag -Wundef.

Reviewed-by: Pavel Reichl <preichl@redhat.com>

resolves:
https://fedorahosted.org/sssd/ticket/1359

last argument of function sss_strnlen "size_t *len" is output variable.
We need to increment value of size_t being pointed to by pointer instead of
incrementing pointer.

* Protect the fd with a mutex when closing
* Set it to a safe value after closing