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 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: 1.6.2;
tests/make: test and explain exporting of variables

tests/make: show a difference between 'export' and '.export'

make: show realistic invalid line in error message

Previously, the error message 'Invalid line' showed only the expanded
line, which might or might not show the actual problem. To be more
helpful, add the unexpanded line to the error message in case they
differ.

Remove the special handling of invalid lines that result from merge
conflicts. RCS is not commonly used anymore, and mentioning CVS was too
specific. By echoing the whole line, the patterns '<<<<<<' and '>>>>>>'
are clear enough to hint at the problem.

make: add more details to error message about invalid lines

tests/make: provide instructions for reproducing bug in gmtime

The test is not enabled because its results differ too much between the
platforms.

make(1): add tests for parsing and exporting variables

Once again, there are a few surprises deeply hidden inside the edge
cases.