Introduce two global variables: `hostname_g' and `interval_g'.

Introduce two global variables: `hostname_g' and `interval_g'.

hostname_g: Hostname we're running on. May be set in the configfile with the
  `Hostname'-option. This replaces the global `hostname'-variable exported by
  `plugin.c'.
interval_g: Interval in which we query the read plugins. This replaces the
  `COLLECTD_STEP'-define and is configurable using the `Interval'-option.