Merge bmake-20210206

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

Merge bmake-20210206

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

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