Fix header file guards

Identifiers starting with "_" are reserved for the C implementation (and/or
the system as a whole, like the kernel).  They aren't for userland stuff.  So
fix the problem.

Also, make the guards upper case, to match common convention.
Cleanup and refactor to_addr_port().

We can now specify either IP addresses (both types!  Country and western!) or
hostnames.  Ports can be either numeric in nature, or textual.  Now that we
have DNS, we can clean up the file and remove unneeded cruft that had
accumulated.
Merge cleanup of to_addr_port().