Attempt to fix crashes with threads in GTK builds

Attempt to fix crashes with threads in GTK builds

* src/xgselect.c (xg_select): Call pselect via thread_select, not
directly, to avoid running Lisp (via unblock_input) when more than
one thread could be running.  (Bug#25247)
* src/process.c (wait_reading_process_output) [HAVE_GLIB]: Call
xg_select directly instead of through thread_select.
* src/xgselect.h (xg_select): Last 2 arguments are no longer
'const', for consistency with thread_select.