DWM process handler

The DWM process handler that allows restarting dwm without closing all windows.

Usage:

1. compile and add "exec dwmguard" to .xinitrc
2. configure a shortcut in dwm that would exit dwm with exit code != 0

The dwm guard will exit if dwm exited with exit code 0 or restart dwm otherwise.

Function to exit with custom exit code:

static void exit_code(const Arg *args)
{
        exit(args->i);
}

Shortcut in DWM configh.h

    { MODKEY|ShiftMask,        XK_r,            exit_code,      {.i = 42} },