Fix warnings in tnftp about invalid constant conversions, e.g.:

contrib/tnftp/src/ftp.c:2067:11: error: implicit conversion from 'int'
to 'char' changes value from 255 to -1 [-Werror,-Wconstant-conversion]
buf[0] = IAC;
~ ^~~
/usr/include/arpa/telnet.h:39:13: note: expanded from macro 'IAC'
#define IAC 255 /* interpret as command: */
^~~
contrib/tnftp/src/ftp.c:2068:11: error: implicit conversion from 'int'
to 'char' changes value from 244 to -12 [-Werror,-Wconstant-conversion]
buf[1] = IP;
~ ^~
/usr/include/arpa/telnet.h:50:12: note: expanded from macro 'IP'
#define IP 244 /* interrupt process--permanently */
^~~

Use an unsigned char buffer instead.

MFC after: 1 week

Copy lukemftp to its new home, tnftp. I'm doing this in stages so as to
not confuse the CVS exporter.