RFC: Install scripts with absolute paths by default

[David Bremner]

I know this is a sortof revert of some changes we have made over the
years, but Linux distros are going to (increasingly?) insist on
absolute paths in shebangs [1]. We could just punt that on to the
distro maintainers, but in the case of Debian that is basically me.

I'm thinking a reasonable compromise is to leave /usr/bin/env in the
source (to allow testing without configure or with a broken
configure), and substitute on install. Thoughts?

If the general approach sounds plausible, then we can move on to
nitpicking my Make/shell

[1]: See e.g. https://pagure.io/packaging-committee/issue/700, or
https://www.debian.org/doc/debian-policy/ch-files.html#scripts