Add TFTP client as specified by RFC 1350

This client does not support the netascii mode.  The default mode
is octet/binary and should be sufficient.

One thing left to do is to check the source port of the server
to make sure it doesn't change.  If it does, we should ignore the
packet and send an error back without disturbing an existing
transfer.

tftp.1

@@ -0,0 +1,29 @@
+.Dd August 14, 2015
+.Dt TFTP 1
+.Os sbase
+.Sh NAME
+.Nm tftp
+.Nd trivial file transfer protocol client
+.Sh SYNOPSIS
+.Nm
+.Fl h Ar host
+.Op Fl p Ar port
+.Op Fl x | c
+.Ar file
+.Sh DESCRIPTION
+.Nm
+is a client that implements the trivial file transfer protocol over
+either IPv4 or IPv6 as specified in RFC 1350.  It can be used to transfer
+files to and from remote machines.
+.Sh OPTIONS
+.Bl -tag -width Ds
+.It Fl h Ar host
+Set the remote hostname.
+.It Fl p Ar port
+Set the remote port.  It defaults to port 69.
+.It Fl x
+Extract a file from the server.  This is the default
+if no flags are specified.  Output goes to stdout.
+.It Fl c
+Create a file on the server.  Input comes from stdin.
+.El