Merge str_substitution branch.

This branch adds additional methods to dialplan functions, whereby the result
buffers are now dynamic buffers, which can be expanded to the size of any
result.  No longer are variable substitutions limited to 4095 bytes of data.
In addition, the common case of needing buffers much smaller than that will
enable substitution to only take up the amount of memory actually needed.
The existing variable substitution routines are still available, but users
of those API calls should transition to using the dynamic-buffer APIs.
Reviewboard: http://reviewboard.digium.com/r/174/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@191140 65c4cc65-6c06-0410-ace0-fbb531ad65f3

funcs/func_dialplan.c

@@ -105,6 +105,7 @@ static int isexten_function_read(struct ast_channel *chan, const char *cmd, char
 static struct ast_custom_function isexten_function = {
 	.name = "DIALPLAN_EXISTS",
 	.read = isexten_function_read,
+	.read_max = 2,
 };
 
 static int unload_module(void)