configs: Updates to sample configs

Includes some minor updates to extensions.conf
and iax.conf. In particular, the demonstration
of macros in extensions.conf is removed, as
Macro is deprecated and will be removed soon.
These examples have been replaced with examples
demonstrating the usage of Gosub instead.

The older exten => ...,n syntax is also mostly
replaced with the same keyword to demonstrate the
newer, more concise way of defining extensions.

IAXTEL no longer exists, so this example is replaced
with something more generic.

Some documentation is also added to extensions.conf
and iax.conf to clarify some of the new expanded
encryption capabilities with IAX2.

ASTERISK-29758 #close

Change-Id: I04fba9671aa1ee9ba1bd5027061f80bbe38e7b46