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.
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 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.
