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: 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: reduce line length in error messages

The error messages for deeply nested parse errors were hard to decipher,
due to the large amount of text. Split these messages into individual
lines, just as in the backtrace for .include files and .for loops. This
unified backtrace makes the output more uniform.

make: in error message about conditionals, use single quotes

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

make: in error messages for anonymous variables, log the value

make: on error, print the targets to be made

This helps to understand situations with several nested sub-makes in
varying directories.

make: add more context information to error messages

In case of a parse error or evaluation error, print the variable value
in addition to the variable name, to see the effects of previous
expression modifiers.

In nested make calls, print the current directory at the bottom of a
stack trace, as that information is otherwise hard to get in a parallel
build spanning multiple directories.

branches: 1.7.2;
make: provide more context information for parse/evaluate errors

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: remove period from end of error messages and warnings

The majority of the existing error messages and warnings does not
include a period at the end. Follow this style consistently.

tests/make: demonstrate use-after-free in modifier ':@'

tests/make: test and document .MAKE.SAVE_DOLLARS and ':='

make: disallow '$' in the variable name of the modifier ':@'

If this restriction should break any existing makefile, the author of
that makefile was probably heading for the IOMCC.

tests/make: split test for modifier ':@' into separate files

The file varmod-loop.mk has grown too large to be single-purpose, plus
it combined parse-time and run-time tests. This has the downside that
as soon as a parse-time test results in an error, the run-time tests are
not run anymore.