Pacify -Wcast-function-type warnings in GCC 8.1

Pacify -Wcast-function-type warnings in GCC 8.1

* src/image.c: Move attributes into DEF_DLL_FN call.
* src/dynlib.c (dynlib_addr): Use get_proc_addr.
* src/w32.h: (get_proc_addr): New function.
(LOAD_DLL_FN): Use it.
(DEF_DLL_FN): Allow function attributes after argument
list.  Add function pointer type used by LOAD_DLL_FN.
* src/w32.c (open_process_token, get_token_information)
(lookup_account_sid, get_sid_sub_authority)
(get_sid_sub_authority_count, get_security_info)
(get_file_security, set_file_security)
(set_named_security_info)
(get_security_descriptor_owner, get_security_descriptor_group)
(get_security_descriptor_dacl, is_valid_sid, equal_sid)
(get_length_sid, copy_sid, get_native_system_info)
(get_system_times, create_symbolic_link)
(is_valid_security_descriptor, convert_sd_to_sddl)
(convert_sddl_to_sd, get_adapters_info, reg_open_key_ex_w)
(reg_query_value_ex_w, expand_environment_strings_w)
(init_environment, create_toolhelp32_snapshot)
(process32_first, process32_next, open_thread_token)
(impersonate_self, revert_to_self, get_process_memory_info)
(get_process_working_set_size, global_memory_status)
(global_memory_status_ex, init_winsock)
(maybe_load_unicows_dll, globals_of_w32): Use get_proc_addr.
* src/w32fns.c (setup_w32_kbdhook, Ffile_system_info)
(get_dll_version, w32_reset_stack_overflow_guard)
(w32_backtrace, globals_of_w32fns): Use get_proc_addr.
* src/w32font.c (get_outline_metrics_w, get_text_metrics_w)
(get_glyph_outline_w, get_char_width_32_w): Use get_proc_addr.
* src/w32heap.c (init_heap): Use get_proc_addr.
* src/w32menu.c (globals_of_w32menu): Use get_proc_addr.
* src/w32proc.c (init_timers, sys_kill, w32_compare_strings):
Use get_proc_addr.
* src/w32uniscribe.c (syms_of_w32uniscribe): Use get_proc_addr.