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.

branches: 1.4.2;
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(1): adjust timestamps in unit tests

These could not be known before the previous commit.

make(1): error out on misspelled directives

Before, make accepted misspellings like .warnings, .export-literally and
a few others, all of which are unlikely to occur in practice. See the
test directive-misspellings.mk for further details.

make(1): add test for misspelled directives

This test allows the other directive-* tests to focus on the purpose of
the individual directive, allowing these tests to continue after
parsing, without errors.