network: Update device statistics (mostly) in the stack.

We bypass device logic in datalink_send_routed_data() in the case
of RTF_LOCAL, so if we don't update the stats there, they'll never
get updated. Furthermore, there's places packets can be dropped
inside the device reader thread. So, we might as well consolidate
the stats-updating logic and get it out of drivers.

(The only remaining case where drivers need to update stats is when
they drop a packet in receive(), as the stack can't tell when an error
from receive() is due to a dropped packet or not.)

Fixes a potential leak on packet drops in the device reader thread,
and fixes loopback statistics for TCP/UDP/etc.

network/ppp: debug trace fixes on x86_64

ppp: Basic build fixes on x86_64

PPP: Port to the new stack.

Signed-off-by: Augustin Cavalier <waddlesplash@gmail.com>

This is a squash of the 42 commits by @mshlyn, as I couldn't find a
way to break them into logical chunks. I did not include these in the build,
as it appears that they only partially work anyway, and much more cleanup is
still needed. However, this is a huge improvement on what was in the tree
before, which looked horrendous and didn't even compile (as it was designed
for the old stack).

Mostly fixes #812.