check_ifstatus: perfdata bug

The perfdata output violates the current Nagios Plugin Development Guidelines
(http://nagiosplug.sourceforge.net/developer-guidelines.html#AEN202 :
"space separated list of label/value pairs").

Thus the Addon PNP did not read the perdata correctly.

The patch replaces the commas with spaces in the perfdata output.

Many thanks to Patric Wust.
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Closes #1169 and #721.