safe_recv(..., len), when used on a blocking fd, will attempt
to call recv and collect data until either EOF, a hard error,
or len bytes are collected.
The previous commit used safe_recv() in a blocking mode to read
a single byte into a buffer that was larger than a byte. This
would cause ndhc to stall as safe_recv() would try to fill that
buffer when no more data would ever be sent.
This issue would only happen if ndhc is supposed to run a script.
Introduce and use safe_recv_once() that will correct this problem and
fill the semantic gap for blocking fds. I add a new call because in
some cases the above behavior might be required for a blocking fd, too.
Note that the above issue is not a problem for nonblocking fds; the
EAGAIN or EWOULDBLOCK path will return a short read.
