A boolean field named GidMappingsEnableSetgroups was added to
SysProcAttr in Go1.5. This field determines the value of the process's
setgroups proc entry.
Since the default is to set the entry to 'deny', calling setgroups will
fail on systems running kernels 3.19+.
Set GidMappingsEnableSetgroups to true so setgroups wont be set to
'deny'.
Signed-off-by: Ido Yariv <ido@wizery.com>