A number of the GSM timers (including T3109) had no reasonable
default values if not specified in the VTY / config file. Together
with unconditional writing to the config file, this created
config files with a persistent setting for important timers as '0'.
To make things worse, many of our example cofig files suffered from the
same problem.
Let's avoid this from happening by
* having reasonable defaults if nothing specified in the config file
* conditionally savingg timers only if they differ from default
* reject any timer values that state zero during start-up (see previous
commit)
Change-Id: Iaac0bfca423852b61d8b9eb1438157ef00d0d8c8
Closes: OS#2380
Harald Welte
5d1fa89819