This is an attempt at getting the sendmail package working. Currently
installing the package does not make the `sendmail` executable appear in
`$PATH`:


> └$ guix environment --ad-hoc sendmail
> └$ sendmail
> bash: sendmail: command not found

This patch fixes that.

Though there are questions that still need to be resolved. While this patch
does fix what it sets out to fix, the solutions aren't neccicerily correct.
It configures a "helpfile" to be put in "$out/usr/share" and the same goes
for its "statistics" file. I have no idea what these does and where they
should go in reality. If someone happens to know, please do tell.