Fix gcc2 warnings.

Fix gcc2 warnings.
Fix the generation/parsing of the Environment option SEND suboption.
Add code (#ifdef ENV_HACK) to help interoperation with Telnet servers
that have the definitions for ENV_VAR and ENV_VALUE reversed.

SCCS-vsn: usr.bin/telnet/Makefile 5.8
SCCS-vsn: usr.bin/telnet/externs.h 5.5
SCCS-vsn: usr.bin/telnet/telnet.c 5.55
SCCS-vsn: usr.bin/telnet/sys_bsd.c 5.4
SCCS-vsn: usr.bin/telnet/commands.c 5.10
SCCS-vsn: usr.bin/telnet/utilities.c 5.7