How to restrict a tcp port to only a specific program (i.e. other program are not allowed access)?
e.g. restrict the tcp port 1600 access to only the myproc program.
Do I use xinetd?
If yes, what is the configuration file setting?

You can't truly restrict it. However by adding an entry to /etc/services for it you can insure it isn't opened by programs that need a random port. Often your issue is simply that something requested a random port and got the one you want.

However if you have two separate programs that are configured to specifically ask for port 1600 then the first one that gets it gets it. The only way to change that is to change the configuration of one of the two programs so that it doesn't request port 1600.

Couldn't you use /etc/hosts.allow? like this:
I think this should only allow programs to listen on ports other than 1600 unless their name is myproc. Doesn't work for clients though...

man pages: hosts_options(5) and hosts_access(5)

I don't know if that would work (not saying it won't - haven't tried it). It doesn't really "reserve" the port though - it just keeps your other programs that try to use it from running.

My point is if you have a program that is configured to explicitly open port 1600 rather than one that is getting it randomly blocking the port isn't really the solution. Sure you might get another program that explicitly opens 1600 working but the first program won't work at all because it requires 1600.

It is much better to try doing the /etc/services setup to prevent random use of port 1600. If you still have something using port 1600 after that you should be able to figure it out with lsof as noted earlier and then you should change one of the competing programs to use a different explicit port (or if you don't need the other to run you should modify your setup so it doesn't try to start rather than blocking it from starting properly. Why waste CPU cycles on something that shouldn't run?