gio/subprocess: Lock while writing on shared pid and status values

The process PID is initialized by the initable vfunc, while g_subprocess_exited sets it again, when we're protecting it via a lock. The status is set when the process exits instead, again while locking.

This makes the thread sanitizer unhappy (even if it shouldn't really be a race for the PID init case), but still locking during initialization is not a bad thing to do.

At the same time g_subprocess_wait() and friends were using the pid and status values without any protection, so let's ensure this is not the case anymore.

WARNING: ThreadSanitizer: data race (pid=8213)
  Write of size 4 at 0x7b200000084c by thread T1:
    #0 g_subprocess_exited ../gio/gsubprocess.c:284
    #1 g_child_watch_dispatch ../glib/gmain.c:5963
    #2 g_main_dispatch ../glib/gmain.c:3373
    #3 g_main_context_dispatch_unlocked ../glib/gmain.c:4224
    #4 g_main_context_iterate_unlocked ../glib/gmain.c:4289
    #5 g_main_context_iteration ../glib/gmain.c:4354
    #6 glib_worker_main ../glib/gmain.c:6553
    #7 g_thread_proxy ../glib/gthread.c:892
  Previous read of size 4 at 0x7b200000084c by main thread:
    #0 g_subprocess_wait ../gio/gsubprocess.c:908
    #1 g_subprocess_wait_check ../gio/gsubprocess.c:939
    #2 end_element ../gio/glib-compile-resources.c:342
    #3 emit_end_element ../glib/gmarkup.c:1045
    #4 g_markup_parse_context_parse ../glib/gmarkup.c:1603
    #5 parse_resource_file ../gio/glib-compile-resources.c:578
    #6 main ../gio/glib-compile-resources.c:967
  Location is heap block of size 120 at 0x7b2000000800 allocated by main
  thread:
    #0 calloc <null>
    #1 g_malloc0 ../glib/gmem.c:133
    #2 g_type_create_instance ../gobject/gtype.c:1933
    #3 g_object_new_internal ../gobject/gobject.c:2621
    #4 g_object_new_valist ../gobject/gobject.c:2960
    #5 g_initable_new_valist ../gio/ginitable.c:245
    #6 g_initable_new ../gio/ginitable.c:163
    #7 g_subprocess_newv ../gio/gsubprocess.c:619
    #8 g_subprocess_new ../gio/gsubprocess.c:590
    #9 end_element ../gio/glib-compile-resources.c:334
    #10 emit_end_element ../glib/gmarkup.c:1045
    #11 g_markup_parse_context_parse ../glib/gmarkup.c:1603
    #12 parse_resource_file ../gio/glib-compile-resources.c:578
    #13 main ../gio/glib-compile-resources.c:967
  Thread T1 'gmain' (tid=8228, running) created by main thread at:
    #0 pthread_create <null>
    #1 g_system_thread_new ../glib/gthread-posix.c:762
    #2 g_thread_new_internal ../glib/gthread.c:996
    #3 g_thread_new ../glib/gthread.c:949
    #4 g_get_worker_context ../glib/gmain.c:6580
    #5 initable_init ../gio/gsubprocess.c:443
    #6 g_initable_init ../gio/ginitable.c:129
    #7 g_initable_new_valist ../gio/ginitable.c:249
    #8 g_initable_new ../gio/ginitable.c:163
    #9 g_subprocess_newv ../gio/gsubprocess.c:619
    #10 g_subprocess_new ../gio/gsubprocess.c:590
    #11 end_element ../gio/glib-compile-resources.c:334
    #12 emit_end_element ../glib/gmarkup.c:1045
    #13 g_markup_parse_context_parse ../glib/gmarkup.c:1603
    #14 parse_resource_file ../gio/glib-compile-resources.c:578
    #15 main ../gio/glib-compile-resources.c:967
SUMMARY: ThreadSanitizer: data race ../gio/gsubprocess.c:284 in
g_subprocess_exited

======================================

WARNING: ThreadSanitizer: data race (pid=15959)
  Read of size 4 at 0x7b200000084c by main thread:
    #0 g_subprocess_wait ../gio/gsubprocess.c:913
    #1 g_subprocess_wait_check ../gio/gsubprocess.c:944
    #2 test_cat_utf8 ../gio/tests/gsubprocess.c:489
    #3 test_case_run ../glib/gtestutils.c:3115
    #4 g_test_run_suite_internal ../glib/gtestutils.c:3210
    #5 g_test_run_suite_internal ../glib/gtestutils.c:3229
    #6 g_test_run_suite ../glib/gtestutils.c:3310
    #7 g_test_run ../glib/gtestutils.c:2379
    #8 main ../gio/tests/gsubprocess.c:2266

  Previous write of size 4 at 0x7b200000084c by thread T1:
    #0 g_subprocess_exited ../gio/gsubprocess.c:284
    #1 g_child_watch_dispatch ../glib/gmain.c:5963
    #2 g_main_dispatch ../glib/gmain.c:3373
    #3 g_main_context_dispatch_unlocked ../glib/gmain.c:4224
    #4 g_main_context_iterate_unlocked ../glib/gmain.c:4289
    #5 g_main_context_iteration ../glib/gmain.c:4354
    #6 glib_worker_main ../glib/gmain.c:6553
    #7 g_thread_proxy ../glib/gthread.c:892

Helps with #3356