runqemu: Fix TAP='TUNSETGROUP: Invalid argument' by falling back to tunctl -u

runqemu: Fix TAP='TUNSETGROUP: Invalid argument' by falling back to tunctl -u

By default the runqemu script tries to set the group permissions on any
tap device it creates.  The TUNSETGROUP ioctl is not implemented on some
popular host enterprise linux distributions.

Internally the script will exit as follows:

++ /opt/qemux86/bitbake_build/tmp/sysroots/x86_64-linux/usr/bin/tunctl -b -g 100
+ TAP='TUNSETGROUP: Invalid argument'
+ STATUS=1
+ '[' 1 -ne 0 ']'
+ echo 'tunctl failed:'
tunctl failed:
+ echo TUNSETGROUP: Invalid argument

This patch implements a fallback to using the userid as the owner of
the tap device which is supported by all 2.6 kernels, the default remains
to try and use the groupid first.

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>