make: simplify debug message for the ':@var@...@' modifier

The previous variant was hard to understand.

make: revert parsing of modifier parts (since 2022-08-08)

The modifier ':@var@body@' parses the body in parse-only mode and later
uses Var_Subst on it, in which each literal '$' must be written as '$$'.

Trying to parse the loop body using Var_Parse treated the text
'$${var:-0}' as a single '$' followed by the expression '${var:-0}',
wrongly complaining about the 'Unknown modifier "-0"'.

Found by sjg.

make: fix parsing of modifiers containing unbalanced subexpressions

make: avoid trailing whitespace in debug log for variables

Since trailing whitespace is invisible, describe the variable value in
words to make it visible.

make: add space after colon in debug logging for variables

These log messages are intended for human interpretation, so don't make
them unnecessarily hard to read.

make: use correct plural form in debug message

make: in debug log, add space between scope and variable name

Without this space, the debug log looked more like line noise, even
though the only punctuation was a single innocent ':'. From a make
user's perspective, the variable name is a word of its own and should
not be visually glued to its namespace.

make: be more verbose in -dv debug logging

The previous log output was too brief to be understandable. Give more
hints by describing each part of the expression when evaluating a
modifier. Distinguish between parse-only mode and eval mode since in
parse-only mode most of the details are irrelevant.

make: remove VarFlags from debug logging

Before the introduction of ExprDefined, VarFlags contained whether the
expression was defined or not, which was useful to know since the final
value of the expression depends on this information. The other VarFlags
do not influence the evaluation, so there is no point logging them.

make: change debug log for variable evaluation flags to lowercase

This makes them easier distinguishable from variable names since the
latter are usually uppercase.

No functional change outside debug mode.

make: rename ExprDefined constants for debug logging

make(1): replace global preserveUndefined with VARE_KEEP_UNDEF

Controlling the expansion of variable expressions using a global
variable and a VARE flag was inconsistent.

Converting the global variable into a flag had to prerequisites:

1. The unintended duplicate variable assignment had to be fixed, as
done in parse.c 1.520 from 2020-12-27. Without this fix, it would have
been necessary to add more flags to Var_Exists and Var_SetWithFlags, and
this would have become too complex.

2. There had to be a unit test demonstrating that VARE_KEEP_DOLLAR only
applies to the top-level expression and is not passed to the
subexpressions, while VARE_KEEP_UNDEF applies to all subexpressions as
well. This test is in var-op-expand.mk 1.10 from 2020-12-28, at least
for the ':@word@' modifier. In ParseModifierPartSubst, VARE_KEEP_UNDEF
is not passed down either, in the same way.

make(1): add test demonstrating the :D modifier with := assignments

make(1): add dummies for fine-grained tests, one per single feature

The test names have been derived from the current manual page.

All these tests are dummies right now, and the code from the existing
tests will be moved into the new tests step by step.

This is done to prevent modmisc, escape, varmod-edge and varmisc from
growing without any bounds, and to reduce the side-effects of one test
to the others.