Makefile ports tweaks cleanup etc

See merge request !211

Netcode fixes the sequel

See merge request !207

Fix save games for a custom mod not saving to custom home folder

See merge request !209

Doing this way because I have no idea if the GP2X port was SUPPOSED to use SDL or not in the first place

Removed all support for Dreamcast, XBox, PSP, PS3, GP2X, Wii, and WinCE from files in the sdl/ folder. If you wanted these ports, use the SDL 1.2 code in sdl12

Also removed GP2X setup from sdl/MakeNIX.cfg, use sdl12 for that too

I think it was originally included for defining the "USEASM" macro, but USEASM isn't used by m_fixed.h/c code anymore

Fixed sdl12/Makefile.cfg and sdl12/(port)/Makefile.cfg and related to refer to sdl12/ subfolders, not sdl/ subfolders

Added a SDL12=1 flag to all ports whose interface code still lives in the sdl12 folder (and is unsupported by SDL 2.0 officially anyway)

This basically causes the makefile to use the sdl12 folder instead of the main sdl folder

SDL2: Relative mouse mode

See merge request !206

P_FloorzAtPos slope fix

See merge request !208

Also, don't post an ev_mouse event_t if not in relative mouse mode, so the camera doesn't jerk when the mouse enters the window

Also did some cleanup and moving around, as well as adding comments