Bug#850967: Clarify /usr/bin/foo should not be hardcoded even in upstream parts

[Sam Hartman <hartmans@debian.org>]

I'll note that the practice of hard-coding paths is fairly common.


One common cause for this is programs that don't want to rely on PATH
for calling exec.  Systemd is a particularly interesting example.
ExecStart and related arguments in systemd units are required to include
full paths.

I am very uncomfortable with the idea of setting policy here.  I find I
tend to agree with Daniel's position a bit more than Ian's.
In particular, I definitely think that for closely-related versions of
software, making sure the same versions are used is helpful.
I've hurt myself more by having parts of something built in /usr/local
than I have not being able to override things for debugging.
However, I
think that both parties have valid points.
So, I'd be much more comfortable if we wanted to help make people more
aware of the tradeoffs than I would setting policy.