Correct regression introduced by previous commit.

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.

scriptd.c

@@ -32,7 +32,7 @@ void request_scriptd_run(void)
         suicide("%s: (%s) write failed: %zd", client_config.interface,
                 __func__, r);
     char buf[16];
-    r = safe_recv(scriptdSock[0], buf, sizeof buf, 0);
+    r = safe_recv_once(scriptdSock[0], buf, sizeof buf, 0);
     if (r == 0) {
         // Remote end hung up.
         exit(EXIT_SUCCESS);