Make the ring buffer a two-state buffer (supplier and consumer)

Make the ring buffer a two-state buffer (supplier and consumer)
rather than a three-state buffer (supplier, sender, and acker).

SCCS-vsn: usr.bin/telnet/defines.h 1.4
SCCS-vsn: usr.bin/telnet/network.c 1.6
SCCS-vsn: usr.bin/telnet/terminal.c 1.6
SCCS-vsn: usr.bin/telnet/telnet.c 5.21
SCCS-vsn: usr.bin/telnet/ring.c 1.4
SCCS-vsn: usr.bin/telnet/ring.h 1.4