make: remove filler word 'Do' from function names for parsing

No functional change, except for debug logging.

make(1): document why the counter tests failed before 2020-09-23

make(1): remove debug output from counter tests

Now that the tests work as intended, the debug information is no longer
necessary. It was only intended to track down the cause of the
unexpected behavior.

make(1): fix unexpected behavior in ::= variable modifier

Previously, the ::= modifier had returned an error value, which caused
the variable expression to be preserved. This behavior was not useful
in this case; it had only been intended to be used for undefined
variables.

To fix it, distinguish between parse errors, undefined variables and
regular empty strings.

make(1): add variant of the counter test

I had expected that using the ::+= modifier instead of the ::= modifier
would work, since the assignment modifier for COUNTER no longer contains
a reference to itself. But instead of ending up at 4, the counter even
goes up to 6.

make(1): add test for nested VAR_SUBST assignments