tickit_debug_init() initialises the libtickit debugging system. It reads the values of the environment
variables and sets up the output filehandle, if enabled. It also has a side-effect on the value of the
tickit_debug_enabled global variable, setting it to the correct value. Applications that wish to read
this variable to control their output of debugging messages may wish to call this function on startup, to
ensure it takes the correct value.
tickit_debug_init() is guarded internally so that it is safe to call multiple times; there is no downside
to opportunistically invoking it on startup anyway, even if some other part of the application may also
have done so.