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
o allow env var MAKE_OBJDIR_CHECK_WRITABLE=no to skip writable
checks in InitObjdir. Explicit .OBJDIR target always allows
read-only directory.
o More code cleanup and refactoring.
o More unit tests
MFC after: 1 week
