Patch for xymonclient-darwin.sh to fix numerous bugs

Greetings,
The following patch fixes these long-standing issues with xymonclient-darwin.sh:

* Disk usage test erroneously uses inode usage values as Darwin combines disk usage and inode usage into the same 'df' output by default.
* Mountpoints with spaces in them were not reported.

The former issues is fixed by passing the '-P' flag to 'df' to restore legacy mode reporting. The latter issue replaces spaces with underscores in the mountpoint.

In addition to checking mount options for 'nobrowse', this patch also exclude readonly volumes which I find are mostly transient DMG files.

Regards,
-Jason

-- 
Jason White
user-bcd1a66a8ba8@xymon.invalid