Update the wpanreatin.sh script (increase compatibility)
Update the wpanreatin.sh script (increase compatibility)

This commit changes the wpanretain.sh script to make in more
compatible with different bash scripts. Also fixes some issues with
parsing/setting of the network-name and xpanid.

abtink/wpantund,aeliot/wpantund,d-bahr/wpantund,d-bahr/wpantund,openthread/wpantund,aeliot/wpantund,openthread/wpantund,openthread/wpantund,aeliot/wpantund,aeliot/wpantund,d-bahr/wpantund,abtink/wpantund,d-bahr/wpantund,abtink/wpantund,abtink/wpantund,openthread/wpantund