workaround for 32-bit systems to handle filesizes bigger than 2GB
OC_Helper::is_function_enabled() added to find out if a function is available
And some documentation adjusted on the way
Fallback to use COM or exec('stat ..') is only executed if stat or filesize return negative values.
Special case of BSD has been added as stat syntax differs
In addition unknown OS is logged
documentation adjusted on the way
enhanced log message
remove duplicate return
Merge pull request #1066 from owncloud/fix-388-master

workaround for 32-bit systems to handle filesizes bigger than 2GB