Allow querying the environment with default values

Add new functions and overloads to the env modules to support querying
environment variables with a default value.
Stop using ATF_WORKDIR in atf-check(1)

Just use TMPDIR instead to determine the location of the temporary file
created by the tool.
Stop using ATF_WORKDIR in atf_check_*

Just use TMPDIR instead to determine the location of the temporary files
created by the atf_check_* familiy of functions.
Remove the ATF_WORKDIR variable

There are no users left behind.
Merge branch 'remove-workdir'

When doing this, update the NEWS file to reflect this change.