BUGFIX: child process counter synchronization

* src/main.c
Avoid race condition on child process counter by blocking SIGCHLD in
main() (while changing counter value).
Revert "BUGFIX: child process counter synchronization"

This reverts commit 1c574af7c4cbe57219110b1caceb1e534a31c11c.

There is a simpler solution. See the next commit. This one will be
saved in a new branch 'sproc_thread'.
Merge branch 'sigprocmask'