refactor: remove dead API, rename to_disk/config_is_remote_dest, fix perror newlines

- Delete unused public array_list_extend (made static)
- Delete legacy 16-parameter parallel_scanner_create wrapper; migrate test to parallel_scanner_create_with_options
- Rename to_disk -> file_write_to_disk and is_remote_dest -> config_is_remote_dest for module_action naming convention
- Remove stray newlines in perror calls (perror already appends one)

src/shared/file.c

@@ -647,7 +647,7 @@ File* receive_incremental_check(int fd, const Config* config, bool* skipped) {
   return file;
 }
 
-bool to_disk(const char* path, const void* data, unsigned long long data_size, bool inplace,
+bool file_write_to_disk(const char* path, const void* data, unsigned long long data_size, bool inplace,
              bool sparse) {
   if (!path || (!data && data_size != 0) || has_path_traversal(path))
     return false;