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.
Interesting changes (from ChangeLog):
o meta.c: ignore paths that match .MAKE.META.IGNORE_PATTERNS
this is useful for gcov builds.
o propagate errors from filemon(4).
o meta.c: apply realpath() before matching against metaIgnorePaths
o allow makefile to set .MAKE.JOBS
