Make the sandbox code allow the writev() syscall.

Tor doesn't use it directly, but the glibc backtrace-to-fd code does
Log a backtrace when the sandbox finds a failure

This involves some duplicate code between backtrace.c and sandbox.c,
but I don't see a way around it: calling more functions would mean
adding more steps to our call stack, and running clean_backtrace()
against the wrong point on the stack.
changes file for 11465
Answer a question in a comment; fix a wide line.
Merge remote-tracking branch 'public/bug11465'