BUG/MINOR: log: make sure writev() is not interrupted on a file output

BUG/MINOR: log: make sure writev() is not interrupted on a file output

Since 1.9 we support sending logs to various non-blocking outputs like
stdou/stderr or flies, by using writev() which guarantees that it only
returns after having written everything or nothing. However the syscall
may be interrupted while doing so, and this is visible when writing to
a tty during debug sessions, as some logs occasionally appear interleaved
if an xterm or SSH connection is not very fast. Performance here is not a
critical concern, log correctness is. Let's simply take the logger's lock
around the writev() call to prevent multiple senders from stepping onto
each other's toes.

This may be backported to 2.0 and 1.9.