telnet_gate() now enables the telnet command pass-through after negotiating
necessary options. This doesn't quite fix the problem reported with telgating
to nethack.alt.org, but it's a step in the right direction.

example, [telnet|rlogin] would set the socket options for either telnet or
rlogin servers, while [telnet] would set the socket options only for the telnet
server.

(replacing sockopts.cfg).

set (previously, TG_CTRLKEYS only worked if TG_LINEMODE was set).

tracks state of local *and* remote option modes to prevent redundant requests
and responses, etc. Should be very robust now.

requests to allow differentiation between received responses (ACK/NAK) and
remote option requests. This solves the problem with Kermit-95 waiting forever
for a response (ACK or NAK) to its initial option requests (i.e. "Negotiate
About Window Size" and "New Environment Option").

"DONT TERMINAL TYPE" Telnet command before passing through traffic from the
remote server (allowing renegotiation of the Terminal Type from the telnet client).

RLogin connections establishment.

this fixes problem with non-blocking socket returning EWOULDBLOCK.

Added TELGATE to telnet_gate() error log messages.

- fixes problem with detecting bad/blank DNS server IP address.

Changed EVENT output_event to sem_t output_sem for POSIX compatibility.