as discussed on the -dev list, introduce a macro to simplify the

as discussed on the -dev list, introduce a macro to simplify the
writing of common constructs like

	chan = !ast_strlen_zero(cdr->channel) ? cdr->channel : "<unknown>";

(or the if/then/else form) into

	chan = S_OR(cdr->channel, "<unknown>");

The name can be changed if we find a better (and not too long) one;
currently, it is S as String, OR as it mimics the behaviour of
the || operator, but applied to strings.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@14747 65c4cc65-6c06-0410-ace0-fbb531ad65f3