build: Fix GCC discarded-qualifiers const errors.

GCC 15.2.1 pays attention to the discarding of the const
qualifier when strchr, strrchr, memchr, or memrchr are now
used. This change fixes numerous errors with this throughout
the tree. The fixes can be broken down into the following:

1. The return value should be considered const.
2. The value passed to strchr or strrchr can be cast as it is
   expected and allowed to be modified.
3. The pointer passed to strchr or strrchr is not meant to be
   modified and so the contents must be duplicated.
4. It was declared const and never should have been.

main/taskprocessor.c

@@ -1174,7 +1174,7 @@ static void *default_listener_pvt_alloc(void)
 static struct ast_taskprocessor *__allocate_taskprocessor(const char *name, struct ast_taskprocessor_listener *listener)
 {
 	struct ast_taskprocessor *p;
-	char *subsystem_separator;
+	const char *subsystem_separator;
 	size_t subsystem_length = 0;
 	size_t name_length;