r15625: Partial commit of my current work. It makes libnet api functions

r15625: Partial commit of my current work. It makes libnet api functions
a bit more smart and more aware of what libnet_context can offer.
The context is a help when some of the arguments are not passed
(programmer counts on using sensible defaults) and stores some of
results so that similar subsequent calls don't need to reopen some
of policy handles, pipes, etc. again. It also helps to hide some
of details the library user don't really want to know much about.

Also, change domain open function to be part of public api, as
it is going to be used in ejsnet interface.

Note, this is work in progress. Comments are welcome.

rafal
(This used to be commit 1ed80c594c2f466e364a11194d6fdc30ac4a8f27)