tftpd: Address flaky tests

The tftpd tests all follow the same pattern:

1. open a UDP socket,
2. fork a child to exec tftpd, which subsequently handles requests on the socket,
3. use a client socket to send some message to the tftpd daemon.

However, tftpd's first action is to mark its socket as non-blocking and
then read a request from it. If no data is present in the socket, tftpd
exits immediately with an error. So, there is a race; we often see
tftpd test timeouts when running tests in parallel. These timeouts also
arise periodically in CI runs.

One solution is to restructure each test to create the server socket,
then write the request to the client socket, then fork tftpd. This
closes the race. However, this involves a lot of churn.

This patch fixes the problem a different way, by adding a new -b flag to
tftpd which makes it block to read the initial request. Each test is
modified to use -b, closing the race.

Reviewed by: imp, asomers
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D47404

(cherry picked from commit 79c342aaf86feb4efbd15383f54e4fe7bdc9da7b)