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 ref

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

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