Fix gcc2 warnings.

Fix gcc2 warnings.
Add ENV_HACK stuff to deal with telnet clients that
have the wrong definitions for ENV_VAR and ENV_VALUE.
Fix up the flow-control processing to work on systems
that don't use Linemode in the server.

SCCS-vsn: libexec/telnetd/Makefile 5.18
SCCS-vsn: libexec/telnetd/pathnames.h 5.6
SCCS-vsn: libexec/telnetd/termstat.c 5.13
SCCS-vsn: libexec/telnetd/utility.c 5.11
SCCS-vsn: libexec/telnetd/state.c 5.13
SCCS-vsn: libexec/telnetd/sys_term.c 5.20
SCCS-vsn: libexec/telnetd/ext.h 5.9