Changes

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.

Related: OS#6947
Change-Id: I46420f4b174d3a9ed01e993e2e89aea3b46c0dd7