Revert "Have mg_adaptor skip over off-lined devices."

This reverts commit 7f29bbd9f4b3826307063ff0cdb66c84c8688ecf.

IT admins would favour the idea not having to have them send USR1
signals to mark a device on-lined once it gets re-installed properly.
To this end the previous behaviour of the mg_adaptor is preserved
except that no error logs about "Connection timed out" for off-lined
devices are generated.
Disable error logging for off-lined devices in mg_adaptor

Have mg_adaptor generate error logging for off-lined devices once
so as to prevent polluting logs. However, the collector thread
on a particular modbus line will still try to read from off-lined
device so that their status can be picked up as soon as they are
on-lined.
Disable signal usage in libcurl to avoid a critical defect

There is a well-known defect in libcurl when using signals to timeout
during DNS lookups resulting in oBIX application unexpected termination
on RHEL6 (but not on Fedora).

The ideal solution is to upgrade libcurl or re-compile it with the c-ares
package which implements async DNS lookup instead of using SIGALM signal.
However, it is discouraged by IT team but to use config option to indicate
whether signals should be disabled in applications configuration file so
that they can tune it accordingly.
Merge pull request #116 from Qingtao-Cao/devel

2nd round of bug fixing for oBIX 1.2.1