Extract unlink_and_reopen from copy_file (#294)

Extract unlink_and_reopen from copy_file (#294)

* add tests to exercise copy_file

* Extract new function unlink_and_reopen from copy_file

The argument `ofd` to copy_file is always set to -1 unless
`open_tmpfile()` is called at generator.c:909

This change
 * removes assignment to a function argument
 * renames argument `ofd` to `tmpfilefd` in line with existing uses
 * extracts a new function `unlink_and_reopen` which is static to util1.c
 * rewrites header comments for copy_file