Update extensions_lib_user.conf

Added the possibility to using RetryDial instead of Dial. You can create a subroutine to use it. An example for every user. More informations about retrydial in the Asterisk CLI with core show application retrydial.

[xivo-subrgbl-user]
exten = s,1,NoOp(Retry Dial)
same  =   n,Set(XIVO_RETRYDIAL=1)
same  =   n,Set(XIVO_RETRYDIAL_ANNOUNCE=hello-world)
same  =   n,Set(XIVO_RETRYDIAL_SLEEP=10)
same  =   n,Set(XIVO_RETRYDIAL_RETRIES=5)
same  =   n,Return()