tests/make: fix typo

tests/make: describe how to read a file into a variable

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: error out on some more syntax errors

Previously, these errors only produced a message on stderr. They only
affected make's exit status when they were evaluated at parse time, but
not when evaluating the commands for a specific target right before
executing them.

The affected syntax errors are:
* invalid regular expressions in the ':C' modifier
* out-of-range references to regex groups in the ':C' modifier
* unfinished modifiers

branches: 1.7.2;
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: make debug logging for comparisons less technical

tests/make: clean up after test for option '-dF'

When the file for the debug log cannot be opened, make exits
immediately. This doesn't give the test a chance to clean up the
temporary log file.

Instead of trying to treat a regular file as a directory and create a
file in it, assume that the directory /nonexistent-$uuid is actually
nonexistent. This leads to the same kind of error message, independent
of strerror(3).

tests/make: test output channels for parse errors and other errors

make: remove usage message if the debug file cannot be opened

Since a non-writable file is not a syntax error, there is no point in
showing the usage in this situation. Showing the usage may have been a
copy-and-paste mistake from a few lines below, when this option was
added back in main.c 1.133 from 2006-10-15.

tests/make: cover a debug log file that cannot be opened

make(1): add tests for each debug option