Merge bmake-20230622

Merge commit '3e39ce563b9ba25883e5aa37d9799eda9e57c1e0'

Merge bmake-20201117

o allow env var MAKE_OBJDIR_CHECK_WRITABLE=no to skip writable
checks in InitObjdir. Explicit .OBJDIR target always allows
read-only directory.

o More code cleanup and refactoring.

o More unit tests

MFC after: 1 week

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.