testing: invoke fake-sendmail the same way sendmail would be invoked

And change it to print its arguments verbatim rather than adding "-t"
to the output.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Pass "-oi" to sendmail by default

The default mail submission method is sendmail.  sendmail defaults to
quitting on a line containing only a ".".  Pass sendmail the -oi flag
to suppress that behavior.  This default can be overridden by
configuring "multimailhook.sendmailCommand" or by instantiating a
SendMailer instance directly.

Suggested-by: Daniel Shahaf
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
README: mention that sendmail is sought in two locations

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>