Allow disabling of netconf notification

[mpereh]

No description provided.

[olofhagsand]

I agree it could be useful to have a knob for turning off netconf notifications.
But I am interested in the usecase, in which way did it fail / why was it necessary?
But there are other ways to design the knob, and this is knob is not right: CLICON_NETCONF_NOTIFICATION_CAP is not defined in clixon-config.yang.
If it is not defined there (and added to the config file) then this will(should) result in a sanity check error when loading the config.
There are also other ways, such as compile time using clixon_custom.h, or make a --enable / --with rule in configure.ac.
One could also define it as a FEATURE possibly.