Fix modbus segfault / New strategy for connecting

Fix modbus segfault / New strategy for connecting

The previous implementation had a reconnection loop which should not
really be necessary. Idea tried here is to check if connection is alive,
and establish it if not. Change was begun because previously connection
would only be attempted as a retry, so on first pass a NULL was handed to
libmodbus which caused segmentation faults reproducibly.

Signed-off-by: Florian Forster <octo@collectd.org>