Allow specifying a custom PATH for which method

Closes #21794.
Make usage of ObserverPathnameExtension more obvious

Remove use of globals.

Closes #21795.
cadubi: fix URL

Fixes #21786.
dpkg: upgrade to 1.17.1 and fix perl scripts

 - Upgrade dpkg to 1.17.1 (most recent release, in debian unstable)

 - Disable dselect and update-alternatives, as they won't work as
   intended in homebrew.

 - Install dpkg Perl modules to $(prefix)/perl, and adjust the shebang
   on Perl scripts to point there using an existing do_perl_subst fn.

Closes #21742.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
mosquitto 1.2

Closes #21769.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
Wine 1.7.0

Closes #21641.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
Check that the cellar exists in Formula.installed

Fixes #21741.
vim 7.4

Closes #21809.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
(gnu) units 2.02

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
gsl 1.16

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
parallel 20130722

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
unrtf 0.21.5

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
xorriso 1.3.2

Closes #21808.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
go: cleanup

 - remove outdated comment
 - remove pointless ENV cleanup
 - remove dead "allow_fail" code
 - remove superfluous parens in block arguments
 - fix indentation
go: fix test
tig 1.2

Closes #21811.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>
Update URL for CLT download

http://connect.apple.com is still down, but
https://developer.apple.com/downloads has all of the Apple developer
downloads available.

Fixes #21812.
Doctor: missed CLT URL replacement