Revert "Process#busy? returns true only when work is about to commence."

This reverts commit 2227cc1804b6822eab1f6a606d054915eaa0141f.
Revert "set `@busy` through SIGUSR1 & SIGUSR2."

This reverts commit 5aff5063be1aa5259ea9d97dff030c77a70f22c8.