bug#42022: --print-data-base should be unambiguous and more easily machine-read

["Daniel Colascione" <dancol@dancol.org>]

It's occasionally useful to parse the output of make --print-data-base,
e.g., for tab completion. Unfortunately, the current (well, 4.2, but I
don't see that it's changed in 4.3) output format prints newlines in
variable definitions verbatim, making it possible for variable contents to
appear (from the point of view of a --print-data-base output parser) to be
makefile directive. For example:

# Begin spoof.mk
define nl


endef
spoof:=$(nl)$(nl)\# Files$(nl)$(nl)foo: bar$(nl)
# End spoof.mk

Make should emit variable variables in a form that cannot be confused with
makefile directives.