This patch updates the shell scripts with some of the common best
practices, which should make them more resilient to unusual failures and
unexpected environments (in particular, directories with spaces).
Most of these were identified by shellcheck.
This patch adds a new integration test, which executes various small
dialogs, to cover corner cases that are not well covered (according to
our coverage report).
For example, "EHLO" without domain, or invalid DATA.
While we could do them via Go tests, this way is more realistic, and the
tests are easier to write.
