CONTRIB: tcploop: add action "X" to execute a command

CONTRIB: tcploop: add action "X" to execute a command

Sometimes it's convenient to be able to execute a command directly on
the stream, whether we're connecting or accepting an incoming connection.
New command 'X' makes this possible. It simply calls execvp() on the
next arguments and branches stdin/stdout/stderr on the socket. Optionally
it's possible to limit the passed FDs to any combination of them by
appending 'i', 'o', 'e' after the X. In any case the program ends just
after executing this command.

Examples :
- chargen server
      tcploop 8001 L A Xo cat /dev/zero

- telnet server
      tcploop 8001 L W N A X /usr/sbin/in.telnetd