MFC r305077:

Squelch clang 3.9.0 warnings about BASE (which is 32768) being converted
to -32768 when it is used as an argument to mp_itom(), in both libtelnet
and newkey. This code has been wrong since r26238 (!), so after almost
20 years it is rather useless to try to correct it.

MFC r305086:

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.