Refactor Easy Netplay scripts.

[XK9274]

Sizeable refactor to the easy netplay scripts, some small behavioural changes.. Mostly structural to make it easier to maintain.
I've tested the scripts w/ Pokemon Red, Blue (GB/GBC scripts) and FireRed / LeafGrean (GB/GBA scripts), Balloon kid (Standard EN Scripts)
A few disconnects across many tests, likely due to the network card as unreproducable and entirely random (Which happened in previous EN versions anyway)
Not tested on Mini Flip as i don't have the devices.

• Added new files easy-netplay_common.sh and easy-netplay_signalling.sh, consolidated duplicate helpers into easy-netplay_common.sh and easy-netplay_signalling.sh.
• Standardized tunables and moved sleep/delay/size constants to a single Tunables section in *common.sh.
• Ensured client confirm panels are gated until the host starts the game (host_ready sentinel).
• Cleaned up comments above functions to match actual behavior.
• Reduced duplicated string processing by centralizing name stripping helpers.
• Added shared env setup file to remove duplicated top-of-file env blocks.
• Centralized cleanup via a shared helper and added per-file removal logging.
• Added file-state logging around save backup/return to aid debugging and trace transfer paths.
• Added network reachability waits before transfers and tweaked hotspot IP acquisition fallback windows.
• Dropped unused stop flag cleanup in the GBA client startup path.
• Skip FTP warmup delay after the first successful HEAD check to reduce repeated sync pauses.

Shared helpers moved into easy-netplay_common.sh

• build_infoPanel_and_log
• checksize_func
• checksum_func
• enable_flag / disable_flag / flag_enabled
• is_running
• restore_ftp
• udhcpc_control
• url_encode
• strip_game_name shared ROM name cleanup
• format_game_name label/prefix-aware naming
• wifi_disabled
• read_cookie optional verbose
• start_ftp optional preflight check_stop message
• check_ftp_local
• ensure_ftp_head
• check_wifi parameterized
• sync_file shared FTP download/verify logic
• create_cookie_info cookie writer with size-gated checksums
• remove_files rm -f with per-file echo
• netplay_cleanup shared cleanup pipeline
• log_file_state quick existence/size/hash logging

Shared helpers moved into easy-netplay_signalling.sh

• notify_peer supports notify_peer or notify_peer
• check_stop supports message arg
• notify_stop
• ready_up
• wait_for_client
• wait_for_client_network
• wait_for_host updated user-facing copy

Shared env setup moved into easy-netplay_env.sh

• sysdir, miyoodir
• LD_LIBRARY_PATH exported
• hostip and peer_ip defaults exported