make: consistently use double quotes in error messages

Previously, some error messages used single quotes, very few used
backticks or parentheses, most already used double quotes.

While here, reword a few error messages to be clearer or more consistent
with related error messages.

tests/make: test expressions based on undefined variables

When an undefined variable is used in a conditional, make complains about
"Malformed conditional", which is wrong since the conditional is
well-formed but errors out during evaluation.

In order to fix this wrong error message and preserve the remaining
behavior, cover the places in which an expression is evaluated with
undefined expressions being an error (VARE_EVAL_DEFINED), but with
neither producing an error message nor handling errors. This combination
results in the same behavior as evaluating the expressions in the mode
that allows undefined variables to be used in expressions.

make: add details to error messages about missing ')'

branches: 1.12.2;
make: clean up comments, code and tests

tests/make: replace 'variable expression' with 'expression'

Each expression is based on a variable, there's no need for the
verbosity. The wording in make's diagnostics will be changed in a
follow-up commit.

tests/make: force line-based diagnostics to be listed in the tests

This way, contradictions between the intended output and the actual
output are closer together and have a better chance of being spotted.

tests/make: migrate cond1 test to other, more specific tests

The tests in cond1 were a mixture of "everything related to conditions",
and the test cases were heavily dependent on each other, which made them
hard to understand. Move each test case to its corresponding
special-purpose test.

make: rename ParseFuncArg to ParseWord, update comments

No functional change.

make(1): clean up some tests

Fix some mistakes in comments.

Improve some comments to be easier understandable.

Inline variables that are irrelevant for the test at hand (RUN,
MAKE_CMD).

Remove references to functions and constants that have since been removed
or renamed.

make(1): add test for parse error in function defined()

make(1): indent directives like .info, .if, .for consistently

Done by "pkglint -F *.mk", no manual corrections.

make(1): normalize whitespace in source code

There is no more space tab. Either only tabs or only spaces or tabs
followed by spaces, but not spaces followed by tabs.

make(1): add test for the function defined(...)

make(1): describe the purpose of each newly added unit test dummy

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.