waitpid for the man process to exit

Without waiting on the forked process, we may check the status of the
wrong $?.

from millert@openbsd

Author: afresh1

lib/Pod/Perldoc/ToMan.pm

@@ -358,6 +358,9 @@ sub _filter_through_nroff {
 		length $done
 		);
 
+	# wait for it to exit
+	waitpid( $pid, 0 );
+
 	if( $? ) {
 		$self->warn( "Error from pipe to $render!\n" );
 		$self->debug( 'Error: ' . do { local $/; <$err> } );