net: context: Operations on unused context should lead to EBADF.

Semantics of ENOENT error as used previously is "named entity not
found", whereas for "I/O handle is not valid", there's EBADF. For
example, POSIX/SUSV2 doesn't even list ENOENT as a possible error
for accept(), connect(), recv(), etc. whereas it lists EBADF, e.g.:
http://pubs.opengroup.org/onlinepubs/7908799/xns/connect.html



Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>