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

contrib/telnet/telnet/commands.c:2914:13: error: implicit conversion
from 'int' to 'char' changes value from 137 to -119
[-Werror,-Wconstant-conversion]

*lsrp++ = IPOPT_SSRR;
        ~ ^~~~~~~~~~

/usr/include/netinet/ip.h:152:21: note: expanded from macro 'IPOPT_SSRR'
#define IPOPT_SSRR 137 /* strict source route */

^~~

contrib/telnet/telnet/commands.c:2916:13: error: implicit conversion
from 'int' to 'char' changes value from 131 to -125
[-Werror,-Wconstant-conversion]

*lsrp++ = IPOPT_LSRR;
        ~ ^~~~~~~~~~

/usr/include/netinet/ip.h:148:21: note: expanded from macro 'IPOPT_LSRR'
#define IPOPT_LSRR 131 /* loose source route */

^~~

Use unsigned char buffers instead.

MFC after: 1 week