include/inst: Allow terminated instances to be cleaned up properly

include/inst: Allow terminated instances to be cleaned up properly

The signal handler that is registered by `_inst_run()' catches the
EXIT and QUIT signals, which causes the handler to be executed one
more time before the process ends. The semaphore that the handler
will attempt to manipulate at that point will have already been
removed, causing the handler to end up in an infinite loop.

This commit changes the trap call so that the EXIT and QUIT signals
are not handled, preventing the signal handler from being executed
more than once.