test-nss-users: add new nss test that resolves users and groups

Inspired by https://bugzilla.redhat.com/show_bug.cgi?id=1929936.
This is similar to test-nss-hosts, but does users, groups, uid, gids.

Functions tested are:
_nss_*_getpwnam_r
_nss_*_getgrnam_r
_nss_*_getpwgid_r
_nss_*_getgrgid_r

Other entry points should be tested too, but it's not relevant to the bug
I was investigating, so I'm leaving that for later ;)

src/basic/nss-util.h

@@ -213,3 +213,25 @@ typedef enum nss_status (*_nss_gethostbyaddr_r_t)(
                 struct hostent *host,
                 char *buffer, size_t buflen,
                 int *errnop, int *h_errnop);
+
+typedef enum nss_status (*_nss_getpwnam_r_t)(
+                const char *name,
+                struct passwd *pwd,
+                char *buffer, size_t buflen,
+                int *errnop);
+typedef enum nss_status (*_nss_getpwuid_r_t)(
+                uid_t uid,
+                struct passwd *pwd,
+                char *buffer, size_t buflen,
+                int *errnop);
+
+typedef enum nss_status (*_nss_getgrnam_r_t)(
+                const char *name,
+                struct group *gr,
+                char *buffer, size_t buflen,
+                int *errnop);
+typedef enum nss_status (*_nss_getgrgid_r_t)(
+                gid_t gid,
+                struct group *gr,
+                char *buffer, size_t buflen,
+                int *errnop);