First network packet transmitted

Using the tests/tnet program:
cat #l/ether0/0/stats
in: 0
link: 0
out: 1
crc errs: 0
overflows: 0
soft overflows: 0
framing errs: 0
buffer errs: 0
output errs: 0
prom: 0
mbps: 10
addr:  0 0 0 0 0 0

Little things. Correct the printf format for the netif device.
Make the file->plan9 sentinal be something other than 1, so we
can maybe detect corruption.
Encapsulate top level waserror() in syswrite. The long term intent
is to not use the waserror() in syscall at all, except as a last
resort backstop and also for testing errstack alignment.
Print the errstr in syswrite, for now, because it's incredibly
handy to see that error text (let us fix something instantly).

Signed-off-by: Ronald G. Minnich <rminnich@gmail.com>