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.