o ignore '.POSIX:' if not in first non-comment line
of Makefile as specified by POSIX.
add unit-tests for above.
Merge commit '92bfae0e6bd53a7a0d6fe55e70a916d86cf26e8b'
Lots of code refactoring, simplification and cleanup.
Lots of new unit-tests providing much higher code coverage.
All courtesy of rillig at netbsd.
Other significant changes:
o new read-only variable .SHELL which provides the path of the shell
used to run scripts (as defined by the .SHELL target).
o variable parsing detects more errors.
o new debug option -dl: LINT mode, does the equivalent of := for all
variable assignments so that file and line number are reported for
variable parse errors.
