MFC r330696, r330709, r330742, r331358

r330696:
Add some functional tests for tftpd(8)

tftpd(8) is difficult to test in isolation due to its relationship with
inetd. Create a test program that mimics the behavior of tftp(1) and
inetd(8) and verifies tftpd's response in several different scenarios.

These test cases cover all of the basic TFTP protocol, but not the optional
parts.

PR: 157700
PR: 225996
PR: 226004
PR: 226005
Differential Revision: https://reviews.freebsd.org/D14310

r330709:
Commit missing file from r330696

X-MFC-With: 330696

r330742:
tftpd: fix the build of tests on i386 after 330696

It's those darn printf format specifiers again

Reported by: cy, kibab
X-MFC-With: 330696

r331358:
tftpd: misc Coverity cleanup in the tests

A bunch of unchecked return values from open(2) and read(2)

Reported by: Coverity
CID: 1386900, 1386911, 1386926, 1386928, 1386932, 1386942
CID: 1386961, 1386979
X-MFC-With: 330696