Changes of interest
o unit-tests: use private TMPDIR to avoid errors from other users
o avoid strdup in mkTempFile
o always use vfork
o job.c: do not create empty shell files in jobs mode
reduce unnecessary calls to waitpid
o cond.c: fix debug output for comparison operators in conditionals
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 variable parsing detects more errors.
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.
