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.
from ChangeLog:
o cond.c: do not eval unnecessary terms of conditionals.
o meta.c: report error if lseek in filemon_read fails
o str.c: performance improvement for Str_Match for multiple '*'
o dieQuietly: supress the failure output from make
when failing node is a sub-make or a sibling failed.
This cuts down greatly on unhelpful noise at the end of
build log. Disabled by -dj or .MAKE.DIE_QUIETLY=no
o unit test for :Or