ccss: Add option to ccss.conf to globally disable it.

ccss:  Add option to ccss.conf to globally disable it.

The Call Completion Supplementary Service feature is rarely used but many of
it's functions are called by app_dial and channel.c "just in case".  These
functions lock and unlock the channel just to see if CCSS is enabled on it,
which it isn't 99.99% of the time.

UserNote: A new "enabled" parameter has been added to ccss.conf.  It defaults
to "yes" to preserve backwards compatibility but CCSS is rarely used so
setting "enabled = no" in the "general" section can save some unneeded channel
locking operations and log message spam.  Disabling ccss will also prevent
the func_callcompletion and chan_dahdi modules from loading.

DeveloperNote: A new API ast_is_cc_enabled() has been added.  It should be
used to ensure that CCSS is enabled before making any other ast_cc_* calls.