client: merged SDL Mouse Button clean up - thx zturtleman@ioq3,

- Make 'b' variable an int so that key values more than 255 will work
(not
currently an issue in ioq3 afaik).
Use SDL_BUTTON_* for easier transition to SDL2.
(SDL2 doesn't have SDL_BUTTON_WHEELUP or SDL_BUTTON_WHEELDOWN, X1/X2 are
different values.)

- some cosmetic & formattings
server: better limits & default values for sv_wh_bbox cvars
refs #268

see
http://dev.etlegacy.com/projects/etlegacy/wiki/Set_up_Features#Wallhack-prediction