Added error checking of kretprobes return values.

No error checking was being done on the return value form copy_process
in fork_handler causing an invalid pointer dereference when copy_process
failed. Also fixes a similar issue in exec_handler.

Thank you to Peter Moody for tracking this down and suggesting a fix.
Remove usage of nf_tproxy_get_sock_v[46] in favor of more direct calls.

In kernel v3.12, features in net/netfilter/nf_tproxy_core.h were moved
to other locations. This commit removes any dependence on that file.
Release v13.12.