MFC r305085:

MFC r305085:

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.

git-svn-id: svn://svn.freebsd.org/base/stable/10@305553 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f