make: use '"filename" line 123' for locations instead of 'filename:123'

The format 'filename:123' is commonly used for identifying a location in
a file. Text editors recognize it and allow quick navigation through it.
The previous format was specific to make and would have required custom
support in editors.

The new format was already used in stack traces, except for the first
line. Now all lines use the same format.

make: replace "Malformed conditional" with "Variable is undefined"

Evaluating a conditional such as ".if ${UNDEF}" produced a "Malformed
conditional" error, even though the form of the conditional was correct,
it was the content of the variable that was wrong.

Replace this message with the more accurate "Variable is undefined",
revealing which of the possibly many variables is actually undefined.

make: omit redundant follow-up message "Malformed conditional"

If there is a parse error or an evaluation error during the conditional,
the conditional is already mentioned in the "while evaluating condition"
line, so there's no reason to repeat it once again.

Same for the "Bad condition" message from the '?:' modifier.

make: in error message about conditionals, use single quotes

Single quotes are used less often in the conditionals themselves, which
leads to fewer confusions.

branches: 1.6.2;
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.

make: don't interpret the return value of Var_Parse

The return value of Var_Parse is largely redundant to the returned
string. The idea behind the type VarParseResult was to migrate all call
sites to checking this return value instead of the returned string, but
that hasn't happened. Instead, the additional type only added more
complexity.

There was a single place where that return value was actually used, when
parsing conditions. And even in that case, ignoring the VarParseResult
added back an error message that previously hid bugs, in the test
cond-token-plain.mk.

Even though these error messages are redundant in the other tests, they
don't hurt as they don't happen often.

make(1): add remarks to the tests about conditions

make(1): remove obsolete comments from tests

make(1): don't require nested variables to be defined in conditions

This code only applies to lint mode (-dL) for now. After a test phase
and a thorough check for possible side effects, it will be activated in
normal mode, too. Having this code in lint mode is required to run
src/build.sh, which relies a lot on using variables with undefined
nested variables in conditions.

In the default mode, any errors about nested undefined variables are not
printed since in Var_Subst, oldVars is true. Therefore, it is not urgent
to properly handle these nested variables correctly there.

make(1): add test for undefined expressions in conditions in lint mode