Import bmake-20210621

Lots more unit tests and code cleanup

Relevant changes from ChangeLog

o job.c: Print -de error information when running multiple jobs
o var.c: only report error for unmatched regex subexpression
when linting (-dL) since we cannot tell when an unmatched
subexpression is an expected result.
reduce memory allocations in the modifiers ':D' and ':U'
reduce memory allocation and strlen calls in modifier ':from=to'
in the ':Q' modifier, only allocate memory if necessary
improve performance for LazyBuf
reduce debug logging and memory allocation for ${:U...}
reduce verbosity of the -dv debug logging for standard cases
fix double varname expansion in the variable modifier '::='
o var.c: avoid evaluating many modifiers in parse only mode
in strict mode (-dL) many variable references are parsed twice,
the first time just to report parse errors early, so we want to
avoid side effects and wasted effort to the extent possible.