Mostly, this changes all the "#if defined(ENCRYPTION)"

Mostly, this changes all the "#if defined(ENCRYPTION)"
lines to "#ifdef ENCRYPTION".  This will allow unifdef
to be used to strip out the encryption code for
foreign exports.  This also fixes up the usage statement.

SCCS-vsn: usr.bin/telnet/authenc.c 5.3
SCCS-vsn: usr.bin/telnet/externs.h 5.7
SCCS-vsn: usr.bin/telnet/telnet.c 5.56
SCCS-vsn: usr.bin/telnet/network.c 5.4
SCCS-vsn: usr.bin/telnet/terminal.c 5.5
SCCS-vsn: usr.bin/telnet/ring.h 5.4
SCCS-vsn: usr.bin/telnet/main.c 5.6
SCCS-vsn: usr.bin/telnet/sys_bsd.c 5.5
SCCS-vsn: usr.bin/telnet/ring.c 5.4
SCCS-vsn: usr.bin/telnet/commands.c 5.12
SCCS-vsn: usr.bin/telnet/utilities.c 5.8