Quite a lot of churn on style, but lots of
good work refactoring complicated functions
and lots more unit-tests.
Thanks mostly to rillig at NetBSD
Some interesting entries from ChangeLog
o .MAKE.{UID,GID} represent uid and gid running make.
o allow env var MAKE_OBJDIR_CHECK_WRITABLE=no to skip writable
checks in InitObjdir. Explicit .OBJDIR target always allows
read-only directory.
o add more unit tests for META MODE
Merge commit '8e11a9b4250be3c3379c45fa820bff78d99d5946' into main
Change-Id: I464fd4c013067f0915671c1ccc96d2d8090b2b9c
