Fix sendmail invocation

[inliquid]

I discovered a couple more issues and I hope this PR will fix them.

1. Interaction with sendmail still "hangs", because Stderr is read before Stdin was closed, should be reversed.
2. Stderr was not read, because io.Reader's Read method does not read into nil slice, according to documentation it will read up to len(p) where p is byte slice.
3. After explicit call to close of Stdin pipe was added, there is no need in deferred Close, plus according to documentation both Stderr and Stdin pipes will be closed automatically when Wait sees program exit.

[wneessen]

Thanks for the PR! Looks good to me.