Unlinked brewed python -> use system python

Up to now a brewed Python, even if not linked, was preferred over
and external Python, so that you had to completely `brew rm python`
to have a formula link against external (mostly system) Python.

From now on it is okay to `brew unlink python`.
unpack: ensure $VERBOSE is restored
unpack: remove dead code
unpack: use ARGV.value
chruby: Fix typo and update dotfiles in caveats.

Closes #21903.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>
SonarQube 3.7

Closes #21901.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>
mksh R48

Closes #21896.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>
transmission 2.82

Closes #21883.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>
sonar-runner 2.3

Closes #21905.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>
sdl2: fix HEAD

Closes #21886.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>
unpack: extract usage
unpack: don't call realpath on potentially non-existent directory
unpack: don't attempt to chdir into regular files