gmain: Make GChildWatchSource child_exited field atomic
gmain: Make GChildWatchSource child_exited field atomic

Ensure synchronization between prepare / check of GChildWatchsource and
UNIX signal dispatcher by making operations on `child_exited` field
atomic. Use `child_exited` as publication flag for `child_status`.

Issue #1312.

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib