* Move the `X-Mailer' header to the top, so that the interesting headers
are all together at the bottom (the top gets littered by server
headers anyway).
* Use `subprocess' directly (`process*' wasn't really doing anything
more than that).
* Allow the sender to be `#f', leaving the header out. This makes all
sendmails that I've used use the username that is running the process.
* Just search for a sendmail program: don't barf on windows, so it can
be used in case there is a sendmail.exe executable there.
* Remove `no-mail-recipients' to make it in-line with other racket code
that doesn't raise super-specific exceptions.
* Use port counting instead of doing the counts manually, much simpler
code.
original commit: f9d07d84009e69a0202390fd828c00d330c19611
