logging: Support nonblocking-io for log target stderr & file, make it default
The nonblocking-io used to be the mode used until recently, where wq only was used if the write buffer was full. This is more performant that always waiting for poll() to write, plus the fact that we write to system synchronously, hence seeing output immediatelly or not losing it if program crashes. Furthermore, It turns out using wq in multithreaded programs may cause problems, so better switch back to using nonblocking-io by default.
