Game : silence some compiler warnings (from gmqcc).
Makefile : changed QC compiler from fteqcc --> gmqcc 0.3.5
Added shared/ folder containing a 'utils.qc' code file.  This will contain
utility functions shared by two or all three programs (MENU, CLIENT, GAME).

Currently utils.qc contains two helper functions for zoned strings.
Client / inventory : added 'invent_id' field to identify inventory slots,
and implemented parsing for "ui_invent" command from the server.
Moved 'screen.qc' file --> shared/
Added 'shared/utils.qc' to all progs.src files.
Client : use new zoned_string() for object description, and tidied up
the CMD_xxx functions e.g. use argv(1) in CMD_describe.
Client : setcursormode(1) -- otherwise getmousepos() does not work
(always returns '0 0 0').
Client : need to setcursormode(0) when in "free camera" mode.