src/utils_cmd_flush.c: Add the `identifier' option.

src/utils_cmd_flush.c: Add the `identifier' option.

Using something like
  FLUSH plugin=rrdtool identifier=localhost/cpu-0/cpu-idle
will now only flush the local cpu-0/cpu-idle value, only using the rrdtool
plugin.

This is meant to be used in frontends which want to display up-to-date graphs
but there are too many hosts/rrd files to update them ``live'', i. e. without
caching.