Exit with badpid for invalid OS PID in fork path

A message sent to a non-existent Unix PID would be discarded, causing
the calling erlang process to block forever in receive.

This change modifies the behaviour of the port to return "badpid" if an
element in the fork path is not found. Since any element of the path may
be invalid, an event (as opposed to a call reply) is generated.
Currently any "badpid" event will cause the erlang process to exit.
Since only one call can be active at a time, it shouldn't interfere with
other operations but the mailbox should pattern match on the list
prefix (i.e., a call to [1,2,3,-1,4] would generate badpid for
[1,2,3,-1]).

Also define the behaviour when writing to invalid PIDs (<= 0). Since 0
is used to initialize the list of child PIDs in the port, passing in a
PID of 0 would match the first available slot in the PID table, causing
the port to write to fd 0.