Update to bmake-20200902

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 change

Update to bmake-20200902

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.

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