[C] Do not fail if Aeron dir exists, but the CnC file does not. (#1481)

If you try to start a C Media Driver with an existing directory,
but without a CnC file inside, it will fail:

ERROR: driver init (2) (2) No such file or directory
[aeron_open_file_rw, aeron_fileutil.c:428] Failed to open file: /dev/shm/empty/cnc.dat
[aeron_driver_init, aeron_driver.c:860] could not recreate aeron dir: /dev/shm/empty

ERROR: driver close (22) (22) Invalid argument
[aeron_driver_close, aeron_driver.c:1117] driver is null

It's not really an error, could be due to the directory being empty,
so it seems better to continue.

This aligns behaviour with the Java Media Driver.

aeron-driver/src/main/c/aeron_driver.c

@@ -204,28 +204,38 @@ int aeron_driver_ensure_dir_is_recreated(aeron_driver_context_t *context)
             aeron_cnc_resolve_filename(dirname, filename, sizeof(filename));
             if (aeron_map_existing_file(&cnc_mmap, filename) < 0)
             {
-                snprintf(buffer, sizeof(buffer) - 1, "INFO: failed to mmap CnC file: %s", filename);
-                log_func(buffer);
-                return -1;
+                if (ENOENT == errno)
+                {
+                    aeron_err_clear();
+                }
+                else
+                {
+                    snprintf(buffer, sizeof(buffer) - 1, "INFO: failed to mmap CnC file: %s", filename);
+                    log_func(buffer);
+                    return -1;
+                }
             }
+            else
+            {
+                snprintf(buffer, sizeof(buffer) - 1, "INFO: Aeron CnC file %s exists", filename);
+                log_func(buffer);
 
-            snprintf(buffer, sizeof(buffer) - 1, "INFO: Aeron CnC file %s exists", filename);
-            log_func(buffer);
+                if (aeron_is_driver_active_with_cnc(
+                    &cnc_mmap, (int64_t)context->driver_timeout_ms, aeron_epoch_clock(), log_func))
+                {
+                    aeron_unmap(&cnc_mmap);
+                    return -1;
+                }
 
-            if (aeron_is_driver_active_with_cnc(
-                &cnc_mmap, (int64_t)context->driver_timeout_ms, aeron_epoch_clock(), log_func))
-            {
-                aeron_unmap(&cnc_mmap);
-                return -1;
-            }
+                if (aeron_report_existing_errors(&cnc_mmap, dirname) < 0)
+                {
+                    aeron_unmap(&cnc_mmap);
+                    return -1;
+                }
 
-            if (aeron_report_existing_errors(&cnc_mmap, dirname) < 0)
-            {
                 aeron_unmap(&cnc_mmap);
-                return -1;
             }
 
-            aeron_unmap(&cnc_mmap);
             if (aeron_delete_directory(context->aeron_dir) != 0)
             {
                 snprintf(buffer, sizeof(buffer) - 1, "INFO: failed to delete %s", context->aeron_dir);