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: 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: in error message about conditionals, use single quotes

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

make: error out on conditions containing the operators '&' and '|'

These abbreviated variants of the '&&' and '||' operators were never
documented, so error out in non-lint mode as well.

branches: 1.5.2;
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.

tests/make: add more comprehensive tests for short-circuit evaluation

tests/make: prevent the bug from cond.c 1.283 from happening again

make(1): add tests for the |, ||, ||| operators

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.