apcups branch: Fix minor issues with the apcups plugin.

apcups branch: Fix minor issues with the apcups plugin.

- Removed `#include <errno.h>' since it is included from `collectd.h'.
- Changed `global_host' back to NULL. Added a ?:-construct to use a default
  value when querying the server.
- Use `strcmp' rather than `strncmp' to compare the full string, not just part
  of the strings.