head(1): refactor main loop

head(1): refactor main loop

The main loop here is horribly obfuscated.  In particular, the path
through the loop to exit(3) is very complex.

Refactor the open/read/write/close portions of the loop out of main()
into a separate function, head_file().

The result is a lot easier to understand at a glance.  In particular,
the path to the end of main() is now dead simple.

Thread: https://marc.info/?l=openbsd-tech&m=164325900400701&w=2

ok millert@