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: 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.

make: add detailed exit status to message for failed sub-commands

Several commands communicate via the exit status and not only
distinguish between zero and non-zero, so make this information
available to ease investigations.

The command "false" is not guaranteed to exit with a consistent status,
so use "(exit 13)" instead in the tests, to keep these tests portable
across different operating systems. The exit status 127 is required for
a shell that cannot find a command, so keep that one.

make: add more context to "returned non-zero status" message

Previously, this message was an "error" but had no influence on the exit
status, so make it a warning instead. In the seldom used -W mode that
treats warnings as errors, this change influences the exit status.

branches: 1.2.4;
make: add debug logging for capturing the output of external commands

This applies to all 4 situations in which the output of an external
command is used for modifying a variable or an expression:

* the assignment operator '!='
* the assignment modifier '::!='
* the SUN shell modifier ':sh'
* the other shell modifier ':!cmd!'

Previously, only the shell modifier ':!cmd!' had debug logging.

Suggested by Christoph Badura.

make: add test for the variable modifier ':sh'