Btest

* Allowing use of -j without a value. BTest then uses the number of
CPU cores as reported by the OS. (Robin Sommer)

* Adding a new "alternative" concept that combines filters and
substitutions, and adds per-alternative environment variables.
(Robin Sommer)

Instead of defining filters and substitutions separately, one now
specifies an alternative configuration to run with "-A <name>" and
that then checks for both "[substitutions-<name>]" and
"[filter-<name>]" section. In addition, "[environment-<name>]"
allows to define alternative-specific environment variables.

The old filter/substitutions options -F and -s are gone. The
sections for substitutions are renamed to "[substitutions-<name>]"
from "[subst-<name>]".

* btest now sets a new environment variable TEST_VERBOSE, giving the
path of a file where a test can record further information about
its execution that will be included with btest's ``--verbose``
output. (Robin Sommer)

* Don't add btest's path to PATH anymore. (Jon Siwek)

* Teach btest's TEST-START-FILE to make subdirectories (Jon Siwek)

* A number of bug fixes, and output polishing. (Robin Sommer)

* More robust background execution by btest-bg-*. (Robin Sommer)