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.
Several misc cleanups, refactor main() to reduce size.
Adds -v option to always fully expand variables (only interesting to NetBSD)
And in particular (from ChangeLog):
o meta.c: ensure command output in meta file has ending newline
even when filemon not being used.
When matching ${.MAKE.META.IGNORE_PATTERNS} do not use
pathname via ':L' since any ':' in pathname breaks that.
Instead set a '${.p.}' to pathname in the target context and
use that.
