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>
Cleanup modbus connection if wrong result

Change-Id: I1b7520442d7663d5bc2529369e27e3e858fe2168
Signed-off-by: Florian Forster <octo@collectd.org>
modbus plugin: Refactor the new reconnection code.

Avoid the goto() and switch() statements.

Github: #294