The read() in readall() to read the SOCKS4a request was sometimes
failing with EAGAIN, which it wasn't prepared for, causing the
request to be rejected and the test to fail.
I wrote this code specifically under the assumption the fd would be
in blocking mode, and in the original draft I wrote with stdin/stdout
under socat that was true. But when I adapted this to do its own
bind/listen/accept logic, I broke it, because POSIX leaves it
unspecified whether accept() inherits the O_NONBLOCK setting or not:
https://pubs.opengroup.org/onlinepubs/9799919799/functions/accept4.html
And the traditional BSD semantics is to inherit O_NONBLOCK.
So, just explicitly clear O_NONBLOCK on the fd returned by accept().
253a001ebc