New netem funtionality and TBF fix

New netem funtionality and TBF fix

New netem-related functionality:
Added ability to save new settings to the kernel. In netem.c, the
netem_get_opts() stub has been replaced with netem_build_msg() which
manipulates the nl_msg data directly and returns an error code instead
of a new nl_msg. Modifications to qdisc_build() in qdisc.c and struct
rtnl_qdisc_ops were necessary for this.
Added support for getting/setting corruption probability/correlation.
Added support for setting a delay distribution.

Fixed tbf_msg_parser() to call tbf_alloc() instead of tbf_qdisc() to
prevent a seg fault.