src, lib: get rid of js errno global

Return error codes directly rather than assigning them to the errno
JS global.

I made node.js core stop doing that in joyent/node@12d0f0b and then
some more in joyent/node@ca9eb71, make unix-dgram follow suit.
src: rename sun -> s

@aklt reported compilation errors on Solaris because of a conflicting
definition of 'sun'.  I've never seen that myself but it doesn't sound
implausible and it's trivial to change.

Hopefully fixes #7.