Add close on exec support for old platforms

Older platfroms like RHEL5 do not have support for O_CLOEXC and need an
explicit fcntl after the fd is created.
Add it conditionally so it can be clearly removed once we declared those
platfroms obsolete and unsupported.

src/sss_client/nss_mc_common.c

@@ -83,6 +83,12 @@ errno_t sss_nss_check_header(struct sss_cli_mc_ctx *ctx)
     return 0;
 }
 
+#ifdef O_CLOEXEC
+#define SSS_MC_OPEN_FLAGS O_RDONLY|O_CLOEXEC
+#else
+#define SSS_MC_OPEN_FLAGS O_RDONLY
+#endif
+
 errno_t sss_nss_mc_get_ctx(const char *name, struct sss_cli_mc_ctx *ctx)
 {
     struct stat fdstat;
@@ -106,10 +112,19 @@ errno_t sss_nss_mc_get_ctx(const char *name, struct sss_cli_mc_ctx *ctx)
         goto done;
     }
 
-    ctx->fd = open(file, O_RDONLY|O_CLOEXEC);
+    ctx->fd = open(file, SSS_MC_OPEN_FLAGS);
     if (ctx->fd == -1) {
         ret = EIO;
         goto done;
+#ifndef O_CLOEXEC
+    } else {
+        int v;
+
+        v = fcntl(ctx->fd, F_GETFD, 0);
+        /* we ignore an error, it's not fatal and there is nothing we
+         * can do about it anyways */
+        (void)fcntl(ctx->fd, F_SETFD, v | FD_CLOEXEC);
+#endif
     }
 
     ret = fstat(ctx->fd, &fdstat);