[PATCH] Improve shell script headers and pre-inst-env handling

[Mark H Weaver]

Hello all,

I've attached two patches.  The first arranges to make sure that
'pre-inst-env' will be rebuilt when 'pre-inst-env.in' is modified.

The second patch is the main subject of this email.  It reworks the
shell script headers at the top of 'guix-package' and the other scripts
to avoid modifying environment variables (which could propagate to
unrelated subprocesses that use libguile), and to avoid prepending
installed directories to the guile load paths in the case where
'pre-inst-env' is being used.

My approach here might be controversial, given that the resulting code
is a bit longer, so if you don't like it, no worries :)

However, I do find it nice to write more scheme and less shell code, and
as a bonus the scheme code at the top is properly handled by emacs and
paredit as if it were in the main body of the file.

What do you think?

      Mark