Remove exim delivery method:

  All sendmail-compatible MTA (exim, but also many others) will work
without any problem with the sendmail delivery method. Implementing a
delivery method for each MTA, using a "non-standard" way, while we could
use the "standard sendmail" way will result on code duplication and
increase maintenance cost. It might also confuses users, making them
think a specific delivery method is required for Exim, while the
sendmail method works as expected.
Revert safe and working shell escaping in sendmail delivery

  Commit 81e9940b0e7a8140ea8178ced03c071c91f756e9 introduced a broken
and less safe escaping mechanism for sendmail command arguments, this
change revert a version more similar as it was just before. Arguments
may be quoted or escaped, but not both.
  We remove example verifying that destinations are quoted, but many
other examples need to be changed back to prior expectations (without
quotes), as they were changed by 81e9940b to force those other tests to
pass.