update sccp.conf example file

The device option is not used in the parsing code.
simplify set callforward in post_line_register_check
use sizeof(*var) instead of sizeof(struct ...)
set session->device to NULL after destroying device

Previously, because we were not setting device to NULL, we were writing
over some previously freed memory.
remove some NULL initialization
use AST_MAX_CONTEXT instead of AST_MAX_EXTENSION

When it's a context, not an extension.
initialize line context with config context
use ast_* alloc/free functions
Merge branch 'misc'