common: more cerr -> derr conversions

common: more cerr -> derr conversions

cmds: cerr -> derr

DoutStreambuf: primitive_log: just write to the stdout fd rather than cerr

assert: don't write output to stderr manually (dout will do that
automatically). Do _dout_lock.Lock rather than TryLock. Failing to wait
for the lock is potentially buggy and provides no benefit in that code.

Signed-off-by: Colin McCabe <colinm@hq.newdream.net>