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.

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: add ":" to error message about unknown modifier

In the manual page, the modifiers are listed with a preceding ":", so
use the same pattern in the error message. This removes an inconsistency
between the error messages "Unknown modifier" and "Bad modifier".

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.

branches: 1.12.2;
make: provide more context information for parse/evaluate errors

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: fix typo in variable name

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: fix reported line numbers of continuation lines (since 2002)

Previously, multi-line directives like '.info' or '.error' reported the
line number of their last line instead of their first line, which is
more usual. This also affected the debug log from '-dp'.

make: improve error message for unknown modifier

Back in 1995, the modifiers were all single-character, and it made sense
to print only the first character. Nowadays, with ':S', ':@var@...@',
'::=' and several others, a little more context is useful to see where
the exact error is. The actual modifier is still guessed, and the guess
may be wrong as soon as backslashes get involved, but it is still better
than before.

make(1): add more tests for tokens in conditionals

make(1): error out on unknown variable modifiers at parse time

Before, make printed an "error message" that did not include the word
error and thus was not easily identified as such. This "error message"
also did not influence the exit status in the default mode but only in
-dL mode. The error message also didn't include any line number
information and was thus rude.

make(1): fix line numbers in test cond-token-string

make(1): add more tests for parsing .if directives

The details of parsing are quite tricky and not documented in the manual
page. Record the current behavior to be a little safer in future
refactorings.

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.