plugin.h: Make DEBUG() a noop when compiling without debugging support.

plugin.h: Make DEBUG() a noop when compiling without debugging support.

This saves a couple of useless calls to plugin_log() which the compiler
does not detect and remove itself.

A couple of DEBUG()'s in the apcups, hddtemp, mbmon and ntpd plugins have
been upgraded to INFO()'s. All of them provide error messages of failed
system / libc calls which should be available to the user somehow.
Besides, they use a local string buffer which generates an "unused
variable" warning if DEBUG() expands to a noop.

Signed-off-by: Sebastian Harl <sh@tokkee.org>
Signed-off-by: Florian Forster <octo@huhu.verplant.org>