SDL2 client fix 'sound' buttons, cleanup network stuff

[s88100]

• fix 'sound' buttons (-5/+5)
• SDL2: use a smaller border around controls in menus and dialogs
• fix 'cleanup network stuff' bug

  
  Thread 1 "pwmangclient" received signal SIGSEGV, Segmentation fault.
  0x0000000800000002 in ?? ()
  (gdb) bt
  #0  0x0000000800000002 in  ()
  #1  0x00005555555b2b23 in Term_xtra (n=10, v=50) at client/ui-term.c:468
  #2  0x0000555555582a55 in Net_cleanup () at client/netclient.c:6468

1 0x00005555555b2b23 in Term_xtra (n=10, v=50) at client/ui-term.c:468

/* Call the hook */
return ((*Term->xtra_hook)(n, v));

2 0x0000555555582a55 in Net_cleanup () at client/netclient.c:6468

Term_xtra(TERM_XTRA_DELAY, 50);

_fixed:_

/ Cleanup network stuff / Net_cleanup() -> Term_xtra(TERM_XTRA_DELAY, 50); -> free_globals(&g_app); ... Free resources ...